Fix fixture matrix derived coverage

[chubes4]

Summary

• replace the fixed 72-fixture policy with one derived active/solved eligibility inventory shared by discovery, preflight, and bench validation
• enforce globally unique stable IDs across active and solved corpora before lane selection; collisions fail closed with corpus, root, and fixture-path attribution
• preserve documented planning_empty behavior for missing, non-directory, and top-level symlink roots while retaining strict validation for real corpus roots
• preserve target-plus-solved and solved-only semantics while preventing duplicate bare IDs from reaching artifact/result paths
• retain coverage evidence in summary.json, cli-run.json, and additive fixture-coverage.json

Evidence

• node --test --test-name-pattern="missing and top-level symlink roots|fixture coverage|active and solved corpora|committed SSI fixture corpus|result evidence writes|builds one-command Blocks Engine|solved-only selects exactly" tools/fixture-matrix.test.mjs (8 passed)
• node --check lib/fixture-matrix/fixtures.mjs && node --check lib/fixture-matrix/result.mjs && node --check tools/run-fixture-matrix.mjs && node --check bench/static-site-fixture-matrix.bench.mjs && node --check tools/fixture-matrix.test.mjs
• git diff --check
• Focused coverage verifies missing and symlink planning-empty roots, addition, deletion/omission, malformed metadata, local and active-vs-solved duplicate IDs, solved-only selection, promotion selection, collision-safe artifact construction, legacy finding-packet consumer compatibility, and the deterministic committed SSI fixture inventory. No fixture matrix execution was run.

Compatibility

• Valid manifest-less fixtures remain discoverable with their existing unknown taxonomy warning.
• finding-packets.json remains the legacy top-level array; no consumer migration is required.
• Coverage is additive in fixture-coverage.json and existing summary/CLI fields.
• Target-plus-solved and solved-only lane selection remain unchanged for globally unique fixture IDs.
• Missing, non-directory, and top-level symlink roots produce the documented planning-empty result instead of throwing during discovery.

Fixes #833

AI Assistance

OpenAI GPT-5.6 Sol via OpenCode implemented the derived coverage contract, review-blocker fixes, docs, and focused tests. Chris Huber reviewed and owns every line.