[codex] Cover runtime semantic plugin validator

[adamziel]

Summary

• add a runtime mu-plugin merge validator to the WordPress semantic COW E2E fixture
• validate the merged plugin-shaped graph through the normal auto-discovered validator path
• keep the validator inert when WordPress loads it outside ForkPress merge-validator env
• document that the E2E clean branch-ID-band case now exercises the plugin-validator contract in both plugin-validator docs and the merge reliability matrix

Why

The semantic E2E already inspected plugin-shaped data after merge, but the product validator path was only covered in PHP unit fixtures. This makes the real runtime path prove that a plugin can validate a coherent graph spanning custom tables, JSON, serialized data, options, postmeta, CPT/page references, and an upload file after a normal branch merge.

This PR is stacked on #49 because current trunk fails before the semantic section at WP UI nonce extraction. Once #49 lands, this should be retargeted to trunk.

Validation

• bash -n tests/cow/e2e.sh
• embedded validator syntax: php -l /dev/stdin
• no-env validator load emits no output
• synthetic coherent-graph validator run under merge-style env returned {\"status\":\"valid\",\"findings\":[]}
• git diff --check origin/codex/wp-ui-nonce-e2e-helper..HEAD