[codex] Extend semantic E2E for Site Editor objects

[adamziel]

What it does

Extends the runtime WordPress semantic merge E2E slice with branch-local Site Editor objects:

• wp_template_part
• wp_template
• wp_global_styles

The test now creates these through WordPress on both source and target branches, stores their IDs in the branch semantic option graph, merges source into target, and verifies both branches' Site Editor rows and option references survived the merge without ID rewriting.

Also updates docs/merge-reliability.md to point the release-gate evidence at the successful v0.1.34 publish run.

Rationale

The fast PHP smoke suite already covers synthetic wp_template_part, wp_template, and wp_global_styles merge cases. This adds the matching runtime coverage so real WordPress insertion, serialization, and branch merge behavior are covered by the semantic E2E path too.

Implementation

The existing forkpress-e2e-semantic.php mu-plugin now inserts Site Editor rows during each branch mutation and includes them in the inspect response. The post-merge validator checks:

1. both source and target rows exist with the expected post types;
2. branch option graphs still point at the correct branch-local Site Editor IDs;
3. template part content, template-part block JSON, and global-styles JSON survive the merge.

This PR is stacked on #313 and should be retargeted to trunk after #313 merges.

Testing instructions

Run:

bash -n tests/cow/e2e.sh
awk '/^cat > "\\$WORK\\/main\\/wp-content\\/mu-plugins\\/forkpress-e2e-semantic.php" <<\\x27PHP\\x27$/{in_block=1; next} in_block && /^PHP$/{exit} in_block{print}' tests/cow/e2e.sh | php -l

Full runtime verification is:

FORKPRESS_E2E_ONLY=semantic tests/cow/e2e.sh <path-to-current-forkpress-binary>

Local full E2E was not run in this worktree because it does not have a built production dist/x86_64-unknown-linux-gnu runtime bundle.