Fix fixture matrix empty-root planning

[chubes4]

Summary

• restore planning_empty for missing, non-directory, and top-level symlink fixture roots before discovery
• preserve derived fixture coverage validation for real executable roots
• align invalid-root direct-bench assertions with Fix fixture matrix derived coverage #834's authoritative requires a complete eligible fixture inventory execution error
• follow up the premature merge of Fix fixture matrix derived coverage #834, which omitted this isolated compatibility fix

Evidence

• composer install --no-interaction --prefer-dist hydrated the PHP dependencies.
• node --test tools/fixture-matrix.test.mjs: 255 passed, 0 failed.
• node --test --test-name-pattern="fixture selection fails closed|top-level symlink fixture roots|missing and top-level symlink roots" tools/fixture-matrix.test.mjs: 3 passed, 0 failed.
• PHP lane-equivalent manifest selection: 37 standalone-PHP tests 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 origin/main...HEAD
• The aggregate npm test runner has an unrelated timing-sensitive failure in the existing runner-progress test at tools/fixture-matrix.test.mjs:5134 when it executes its Node suite alongside the manifest; the same full file passes standalone as above.
• No fixture matrix execution was run.

Compatibility

• Existing invalid-root dry-run behavior is restored rather than throwing before the planner can report exclusions.
• The wrapper CLI continues to report its established planning-empty handoff message. Direct bench execution consistently reports the eligible-inventory contract.
• Real fixture roots retain malformed-metadata, duplicate-ID, and omission validation from Fix fixture matrix derived coverage #834.
• The change is limited to fixture discovery and focused test-contract coverage.

Refs #833
Follow-up to #834

AI Assistance

OpenAI GPT-5.6 Sol via OpenCode rebased/cherry-picked the isolated fix, aligned the stale assertions, hydrated dependencies, ran focused/full Node and PHP-equivalent validation, and drafted this PR update. Chris Huber reviewed and owns every line.