[compat] DuckDB unsupported metadata source freshness parity

[sabino]

Objective

Implement the first safe child slice from #206: DuckDB metadata-only source freshness should fail with a dbt-grounded unsupported metadata runtime-error row instead of inventing DuckDB metadata freshness behavior.

Current Reality

#206 established that dbt Core can use adapter metadata freshness only when the adapter advertises TableLastModifiedMetadata / related metadata APIs. The current dbt-duckdb path does not expose a safe upstream-backed metadata freshness implementation. dxt already supports DuckDB loaded_at_field, loaded_at_query, freshness inheritance, relation identity, and Sources v3-shaped success/runtime-error output.

Role and Labels

• role: role:worker
• labels: type:compat, area:parser, area:artifacts, area:duckdb, artifact:sources, command:source-freshness
• readiness: status:ready, ready-for-agent

Priority

priority:p1: source freshness artifacts feed selector behavior and public fixture compatibility.

Dependencies / Sequencing

• depends_on=[compat] Source metadata freshness reference map and first slice #206
• Use the Reference Map and Slice Plan comment on [compat] Source metadata freshness reference map and first slice #206 as the source-grounded specification.
• Do not change source_status:fresher, selector semantics, or adapter ABI design in this issue.

Acceptance Criteria

• For selected DuckDB sources with freshness but neither loaded_at_field nor loaded_at_query, dxt source freshness emits a Sources v3 runtime-error row with a dbt-grounded unsupported metadata message.
• Other selected sources with loaded_at_field / loaded_at_query still run and emit success/warn/error rows as before.
• No DuckDB metadata execution is invented from file mtimes, information_schema, local state, or private dxt fields.
• Runtime behavior remains Zig-only.

Validation

• native Zig test in src/project/source_freshness.zig for the unsupported metadata reason/writer boundary
• focused pytest through the Zig binary for a metadata-only DuckDB source freshness fixture
• Sources v3 schema slice validation
• python scripts/check_runtime_boundary.py
• python scripts/check_public_safety.py

Stop Condition

Stop and split if the implementation requires a general adapter capability matrix, get_relation_last_modified, non-DuckDB adapter behavior, source-status selector changes, or any Python product runtime behavior.

[sabino]

Agent Claim

• Role: dxt_zig_slice_worker
• Branch: agent/issue-210-compat-duckdb-unsupported-metadata-source-freshn
• Mode: implement
• Claimed at: 2026-06-21T18:07:17Z