[codex] Cover stale term parent semantic references

[adamziel]

What it does

Adds a focused WordPress semantic validator regression for hierarchical taxonomy parent references. The fixture covers a source branch deleting a parent category while the target edits the child term and child taxonomy row that still point at that parent.

Rationale

wp_term_taxonomy.parent stores a WordPress term relationship, not just a scalar value. A row-level merge can preserve the target child edits and apply the source parent delete, but the resulting taxonomy graph is incoherent unless a semantic validator reports the stale parent reference for review.

Implementation

The new discovered validator scans wp_term_taxonomy rows with parent > 0, verifies that the parent term still has taxonomy for the same taxonomy name, and emits plugin-wp-term-parent-missing-reference findings. The test asserts that the source parent term/taxonomy deletion remains staged, target child edits remain visible, and plugin audit output identifies the missing parent term, child taxonomy row, and stale parent field.

The merge reliability matrix now records term-parent reference coverage alongside existing term relationship coverage.

Testing instructions

php -l tests/cow/wp_semantic_validator.php
make test-cow-wp-semantic-validator
make test-cow-semantic-fast
npm run check
git diff --check