docs(tools-pack): add build cache contract

AGENTS.md

@@ -11,6 +11,7 @@ This file is the single source of truth for agents entering this repository. Rea
 - References and current plans: `docs/references.md`, `docs/code-review-guidelines.md`, `specs/current/maintainability-roadmap.md`, `specs/current/ci.md` (CI scope confidence methodology — required before changing confidence or guard fields in `scripts/scopes.ts`).
 - Directory-level agent guidance: `.github/AGENTS.md`, `apps/AGENTS.md`, `packages/AGENTS.md`, `tools/AGENTS.md`, `e2e/AGENTS.md`.
 - Packaged auto-update architecture and high-confidence local harness: read `tools/pack/AGENTS.md` section "Packaged auto-update architecture and harness" before touching packaged updater code, release-channel identity, installer behavior, or updater UI.
+- Packaged build cache contract: `tools/pack/CACHE.md` (determinant rules, materialization-time parameters, confidence grading — required before changing any build-cache node key).
 
 ## Workspace directories
 

tools/pack/AGENTS.md

@@ -2,6 +2,8 @@
 
 Follow the root `AGENTS.md` and `tools/AGENTS.md` first. This tool owns the repo-external packaged build/start/stop/logs command surface.
 
+Read `tools/pack/CACHE.md` before changing any build-cache node key, adding a cache node, or changing what a cached node reads or writes. It is the source of truth for the build-graph cache under `--cache-dir`: determinant rules, materialization-time parameters, the signing boundary, and confidence grading.
+
 ## Owns
 
 - Local packaging orchestration for packaged Open Design artifacts.

tools/pack/CACHE.md

@@ -0,0 +1,175 @@
+# tools-pack build cache contract
+
+This file is the source of truth for the `tools-pack` build-graph cache: the
+cache under `--cache-dir` that stores packaged build artifacts as
+content-addressed nodes. Read it before changing any node key, adding a node,
+or changing what a node writes.
+
+It records **current state only**, not change history — same convention as
+`specs/current/ci.md`.
+
+Out of scope: the GitHub Actions cache that wraps this store. That layer is
+owned by `.github/`; it restores and saves the store as an opaque directory and
+carries no correctness obligation (see **Why coarse restore is safe** below).
+
+## Cache model
+
+A node is `{ id, key, outputs, build, invalidate }`, acquired through
+`ToolPackCache` (`src/cache.ts`).
+
+- `keyHash = hash(node.id + "\n" + node.key)`; the entry lives at
+  `entries/<node.id>/<keyHash>/` with a `manifest.json`.
+- An entry is accepted only when `manifest.key === node.key` exactly. A
+  mismatch is reported as `key mismatch` and the node rebuilds
+  (`cache.ts:270-282`). There is no fuzzy or prefix matching inside the store.
+- `invalidate` is an additional per-node veto applied to an otherwise-valid
+  entry (for example `win.packaged-app` re-validates its native rebuild
+  output).
+- `materialize` copies entry outputs to their workspace locations. Steps that
+  run on the materialization path execute on **both** the hit and the miss
+  path.
+
+### Why coarse restore is safe
+
+Because acceptance is an exact `node.key` comparison, a restored store is a
+*pool of candidates*, not an authority. Entries that do not match are ignored.
+An outer layer may therefore restore a broader or older store than strictly
+requested without risking a wrong build — it only affects how much work is
+skipped, never what is produced.
+
+This property is what the surrounding CI caching depends on. Do not weaken it
+by introducing prefix or best-effort matching inside the store.
+
+## Node inventory
+
+The build-graph cache is almost entirely Windows-specific.
+
+| Node | Platform |
+| --- | --- |
+| `<platform>.workspace-build` | all |
+| `win.resource-tree` | win |
+| `win.workspace-tarballs` | win |
+| `win.packaged-app` | win |
+| `win.electron-builder-dir` | win |
+| `win.nsis-payload-base` | win |
+| `win.nsis-payload-overlay` | win |
+| `win.nsis-installer` | win |
+| `win.portable-zip` | win |
+| `win.launcher-payload-base` | win |
+| `win.launcher-payload` | win |
+
+`mac` and `linux` have `<platform>.workspace-build` only.
+
+## Determinant rules
+
+**R1 — A node key must cover every input that determines the node's output.**
+Inputs include file content, configuration values, tool versions, and process
+environment. An input that is read by `build` but absent from `key` is a
+defect, not an optimization.
+
+**R2 — A node key must carry the key of every upstream node it consumes.**
+Re-deriving an upstream node's own inputs is not a substitute: upstream keys
+carry inputs that are not file content (see R3), so re-derivation silently
+drops them.
+
+Existing links: `win.packaged-app` carries `tarballsKey`;
+`win.electron-builder-dir` carries `packagedAppKey` and `resourceTreeKey`;
+`win.nsis-installer` carries `basePayloadKey` and `overlayPayloadKey`;
+`win.launcher-payload` carries `sourceKey`.
+
+**R3 — Build outputs are never direct key inputs.** `hashPackageSourcePath`
+excludes `dist`, `.next`, `out`, `node_modules`, and `.od`. A node that
+consumes another node's build output must obtain that output's identity
+through R2, not by hashing the output tree.
+
+**R4 — A node key must not restate a list that already exists as a
+constant.** Derive key inputs from the constant instead. Two independent
+restatements of the same list drift silently and produce stale entries with no
+signal.
+
+**R5 — Every declared input needs a witness.** See **Changing a cache node**.
+
+## Materialization-time parameters
+
+Some values deliberately do **not** enter node keys. They are stamped onto the
+output every time the node is materialized, so cached content stays
+parameter-agnostic and is specialized on the way out.
+
+Current materialization-time parameters:
+
+- **App version.** `win.packaged-app` omits it. It is applied through
+  electron-builder `extraMetadata.version`, then rewritten on materialization
+  by `rewriteUnpackedAppPackageVersion` and `rewriteWinExecutableVersion`, then
+  verified by `assertMaterializedUnpackedVersionConsistency` — a fail-closed
+  check over the app `package.json` version, the `open-design-config.json`
+  `appVersion`, and the Windows executable fixed file version.
+- **Namespace / channel and runtime endpoints.**
+  `win.electron-builder-dir` omits them. `open-design-config.json` — which
+  carries `namespace`, `amrProfile`, `telemetryRelayUrl`, `updateMetadataUrl`,
+  `posthogKey`/`posthogHost`, `webOutputMode`, and `namespaceBaseRoot` — is
+  regenerated on the materialization path by `writePackagedConfig`.
+
+Downstream archive nodes (`win.nsis-*`, `win.portable-zip`,
+`win.launcher-payload*`) carry `namespace` and the full `packagedVersion` in
+their keys, because their content is the already-stamped payload.
+
+**Requirement.** A value may be a materialization-time parameter only when
+both hold:
+
+1. it is re-applied unconditionally on the materialization path, so the hit
+   and miss paths converge; and
+2. the applied value is verified by a fail-closed assertion.
+
+Adding a materialization-time parameter without (2) is not permitted.
+
+> Known asymmetry: app version satisfies (2); namespace currently satisfies
+> only (1) — it is rewritten but not asserted.
+
+## Signing boundary
+
+Signing material never enters cache content. `resolveWinSigningCacheKey`
+(`src/win/sign.ts`) contributes only the certificate SHA-1, digest algorithm,
+timestamp algorithm, and timestamp URL, and appears in the keys of
+`win.nsis-payload-overlay`, `win.nsis-installer`, and `win.portable-zip`.
+
+`win.nsis-payload-base` correctly omits signing: it is built before
+`ensureSignedUnpacked()`, while the overlay is built after. Keep that ordering
+when changing the payload split.
+
+## Confidence tiers
+
+Borrowed from `specs/current/ci.md`.
+
+- **`certain`** — the key provably covers the node's full input closure, and a
+  witness test demonstrates it (see below). Only `certain` nodes may
+  participate in cross-run cache reuse policies built on top of this store.
+- **low confidence** — anything else.
+
+**Grading is fail-closed: a node is low confidence until a witness proves
+otherwise.** A newly added node is low confidence by default.
+
+## Declared low-confidence points
+
+Recorded so that grading stays checkable. These are known and accepted; do not
+extend them.
+
+- `<platform>.workspace-build` — `pnpm-workspace.yaml` is not a key input;
+  file mode (executable bit) is not hashed by `hashPackageSourcePath`.
+- `win.launcher-payload` — the `seed: "nsis-base"` branch takes content from
+  the NSIS base payload but carries only the literal `"nsis-base"`, not
+  `WIN_ARCHIVE_CACHE_VERSION`. Bumping that constant without bumping the
+  launcher payload cache versions mismatches.
+
+## Changing a cache node
+
+1. Read this file and `AGENTS.md` in this directory.
+2. If the change alters what `build` reads or writes, update the key in the
+   same commit.
+3. Bump the node's `schemaVersion` / cache-version constant whenever key
+   semantics change.
+4. Add or update the node's witness test in `tests/`. A witness proves both
+   halves:
+   - mutating **each declared input** changes the key;
+   - mutating a **known non-input** (for example a package's `dist` tree)
+     leaves the key unchanged.
+5. Never introduce prefix or best-effort matching inside the store.