Add first state:new selector slice

[sabino]

Summary

• Adds a Zig prior-manifest state reader for Manifest v12 resource unique IDs.
• Supports state:new through the shared selector engine, including graph expansion and --exclude composition.
• Adds focused CLI coverage, compatibility docs, changelog, and PLAN status updates.

Linked Issue

Closes #211

Scope

• In scope: state:new membership against --state/manifest.json, deterministic missing/malformed/version errors, graph expansion, and exclude composition.
• Out of scope: state:modified, state:old, state:unmodified, deferral, checksum comparison, package install state, partial parsing, and Python product runtime behavior.

Source Grounding

• dbt Core v1: StateSelectorMethod / PreviousState behavior as mapped in the existing YAML selector and state/defer roadmap.
• Fusion / v2: selector/state references from the same roadmap remain non-overriding context.
• Not applicable because: this is a narrow current-vs-prior unique-id membership slice, not a broader state diff or defer implementation.

dxt Ownership

• Zig modules: src/project/state.zig, src/project/selector.zig, src/project.zig, src/root.zig.
• Artifacts: reads prior manifest.json; no generated artifact shape changes.
• Commands: selector-backed commands, with focused validation through dxt ls.

Validation

• zig build
• zig build test with explicit temp cache
• Focused pytest: pytest -q tests/test_cli.py -k 'state_new' (2 passed, 1 optional dbt oracle skipped)
• Focused selector regression pytest: pytest -q tests/test_cli.py -k 'source_status_selector or result_selector or yaml_selector or repeated_selector_flags' (5 passed, 2 optional dbt oracles skipped)
• Public Jaffle gate
• python scripts/check_runtime_boundary.py
• python scripts/check_public_safety.py
• Not applicable because: public Jaffle gates are outside this selector/artifact slice.

Agent Evidence

• Role: dxt_zig_slice_worker
• Branch: agent/issue-211-compat-first-state-new-selector-from-prior-manif
• Worktree scope (no local path): issue [compat] First state:new selector from prior manifest #211 selector/artifact slice only.
• Specialist review: not requested.
• Reflection required: no.

Risks

• Runtime boundary: product behavior remains Zig-only; Python changes are CLI tests only.
• Public safety: scan passed.
• Artifact parity: first slice only validates Manifest v12 resource maps and unique IDs; no checksum diffing.
• Branch overlap: rebased on origin/main; branch is one commit ahead.
• Follow-up: broader state selectors and defer remain planned.