Execute supported custom generic tests

[sabino]

Summary

• Enables the already-supported static custom generic-test subset to execute through DuckDB for dxt test and dxt build.
• Emits Run Results v6-shaped pass/fail/warn rows and deterministic error rows for supported custom generic-test DuckDB execution failures.
• Updates compatibility docs, changelog, and PLAN status for the new runtime boundary.

Linked Issue

Closes #212

Scope

• In scope: root-project and installed-package model/source/seed column custom generic tests whose macros are static {% test %} / {% data_test %} blocks using {{ model }} and {{ column_name }}.
• In scope: existing generic-test where, limit, severity thresholds, store_failures, failure-count wrapping, and Run Results shape for the supported custom subset.
• Out of scope: table-level or non-column custom generic tests, broader Jinja, adapter dispatch, store_failures_as, fail_calc, SQL headers, custom materializations, bundled dbt internals, and Python product runtime behavior.

Source Grounding

• dbt Core v1: focused pytest includes a local dbt Core/dbt-duckdb oracle comparison for the simple custom-test fixture when those dependencies are importable.
• Fusion / v2: not used in this runtime slice.
• Not applicable because: the slice is bounded to the existing dbt Core-compatible static generic-test subset already compiled by dxt.

dxt Ownership

• Zig modules: src/project.zig, src/project/compiler.zig, src/project/duckdb.zig, src/project/run_results.zig, src/root.zig.
• Artifacts: Manifest test metadata namespace consistency for source custom tests, compiled custom test SQL, Run Results v6 rows.
• Commands: dxt test, dxt build.

Validation

• zig build
• zig build test with explicit isolated Zig caches: 305/305 tests passed.
• Focused pytest: python -m pytest -q tests/test_cli.py -k custom_generic passed, 9 passed / 208 deselected, including the local dbt oracle path where available.
• Public Jaffle gate
• python scripts/check_runtime_boundary.py passed.
• python scripts/check_public_safety.py passed.
• zig fmt --check src/project.zig src/project/compiler.zig src/project/duckdb.zig src/project/run_results.zig src/root.zig passed.
• git diff --check passed.
• Not applicable because:

Note: exact default-cache zig build test --summary all currently fails immediately in this worktree with PermissionDenied. The same command semantics pass with explicit local/global cache directories, so this is recorded as local Zig cache state rather than a code failure.

Agent Evidence

• Role: role:worker.
• Branch: agent/issue-212-compat-execute-simple-custom-generic-tests-throu.
• Worktree scope (no local path): issue [compat] Execute simple custom generic tests through DuckDB #212 branch only; no open PR overlap found at startup; [compat] Execute parsed unit tests for the first DuckDB fixture slice #205 blocker had landed before implementation.
• Specialist review: not requested.
• Reflection required: no, scope stayed within the issue stop conditions.

Risks

• Runtime boundary: execution, classification, and Run Results emission remain in Zig; Python changes are test/oracle coverage only.
• Public safety: public-safety scan passed.
• Artifact parity: custom error rows are deterministic and Run Results v6-shaped, but broader dbt custom-test runtime semantics remain out of scope.
• Branch overlap: issue [compat] Parse and list snapshot resources as read-only manifest nodes #213 later ran a separate parser/snapshot worker; this branch does not touch snapshot parser/selector code.
• Follow-up: broaden custom generic-test semantics only through separate slices with explicit dbt source grounding.