Handle unsupported DuckDB metadata freshness

[sabino]

Summary

• Return a dbt-grounded unsupported metadata freshness reason for DuckDB sources with freshness but no loaded_at_field or loaded_at_query.
• Keep Sources v3 runtime-error rows for those sources while continuing selected loaded-at-field and loaded-at-query freshness checks.
• Update focused CLI coverage plus compatibility, plan, and changelog notes.

Linked Issue

Closes #210

Scope

• In scope: DuckDB metadata-only source freshness unsupported runtime-error behavior and focused Sources v3 schema coverage.
• Out of scope: DuckDB metadata freshness execution, adapter capability matrices, get_relation_last_modified, non-DuckDB behavior, source-status selector changes.

Source Grounding

• dbt Core v1: [compat] Source metadata freshness reference map and first slice #206 Reference Map cites core/dbt/task/freshness.py::FreshnessRunner.execute, source parsing warnings, adapter capabilities, and Sources v3 runtime-error shape.
• Fusion / v2: [compat] Source metadata freshness reference map and first slice #206 Reference Map cites freshness artifact structs and metadata freshness adapter hooks.
• Not applicable because: real metadata execution remains unsupported for DuckDB in this slice.

dxt Ownership

• Zig modules: src/project/source_freshness.zig, src/project.zig.
• Artifacts: sources.json Sources v3 runtime-error rows.
• Commands: dxt source freshness.

Validation

• zig build using isolated ignored cache paths because the default local Zig cache hit a mount rename PermissionDenied.
• zig build test full suite: attempted, interrupted after extended silence inside the Zig test runner on the busy worker host.
• Focused native Zig test: zig test src/project/source_freshness.zig --cache-dir .agent/runs/issue-210-zig-test-cache/local --global-cache-dir .agent/runs/issue-210-zig-test-cache/global.
• Focused pytest: python -m pytest tests/test_cli.py::test_source_freshness_reports_unsupported_duckdb_metadata_and_continues -q.
• Public Jaffle gate: not needed for this unsupported-DuckDB diagnostic slice.
• python scripts/check_runtime_boundary.py.
• python scripts/check_public_safety.py.
• git diff --check and zig fmt --check src/project/source_freshness.zig src/project.zig.

Agent Evidence

• Role: dxt_zig_slice_worker.
• Branch: agent/issue-210-compat-duckdb-unsupported-metadata-source-freshn.
• Worktree scope (no local path): issue [compat] DuckDB unsupported metadata source freshness parity #210 only; source freshness Zig/artifact/test/docs slice.
• Specialist review: not requested.
• Reflection required: no.

Risks

• Runtime boundary: product behavior stays in Zig; Python change is focused black-box CLI/schema test coverage.
• Public safety: scan passed.
• Artifact parity: runtime-error row remains Sources v3-shaped with only unique_id, error, and status.
• Branch overlap: rebased on current origin/main before final validation and push.
• Follow-up: real metadata freshness execution still needs an explicit Zig adapter capability/API design and a safe upstream-backed adapter implementation.