feat(ingest): PriorityGate — RFC-003 Phase 1b (logic only, not wired)

[0xmanhnv]

Pure priority-gate logic for asset-source precedence. Stateless, deterministic, fully unit-tested — but NOT yet wired into the ingest hot path. Phase 1b2 will integrate with
processor_assets.mergeCTISIntoAsset once per-field attribution data (Phase 1d) is populated in asset_sources.contributed_data.

Ship-the-logic-first approach rationale:

1. Gate algorithm is non-trivial (combined Priority + TrustLevels ranking, ownership semantics, tie-break rules from RFC-003). Reviewable in isolation with a 12-case test matrix.
2. Integration is the risky part — touches ingest hot path. Better to validate the algorithm on its own before touching 1,000+ assets/sec ingest throughput.
3. Standalone file, zero runtime cost, easy to revert if customer answers on open questions mandate changes.

What's in:

• PriorityGate interface with CanWrite + FilterProperties
• defaultPriorityGate implementation (stateless, share-safe)
• Reason constants (feature_disabled, unowned_field, same_source, higher_or_equal_rank, lower_rank) — used by metrics + audit
• rankOfSource() combines Priority list position + TrustLevels buckets into a single comparable integer; listed sources always outrank TrustLevel-only sources (Q2 rule)

Test matrix (12 cases, all passing):

• Feature off → all writes allowed (regression guard)
• Unowned field → always allowed (new knowledge never blocked)
• Same source → self-overwrite allowed
• Listed beats listed by position
• Listed beats unlisted (Q2)
• TrustLevels outrank each other (Primary > Low)
• Equal rank → allow (Q8 tie-break goes to last-write-wins)
• Priority wins over TrustLevels advisory
• Position is stable (off-by-one regression guard)
• FilterProperties splits allowed vs skipped
• Feature-off fast path returns input unchanged (zero alloc)
• Nil incoming map handled safely

What's NOT in:

• Wiring into processor_assets.go — Phase 1b2
• Writing asset_sources.contributed_data.fields on each ingest — Phase 1d
• Redis cache for tenant settings — Phase 1b3 (perf optimization)
• Skip-audit API (GET /assets/{id}/source-skips) — Phase 1d