[codex] Merge independently inserted theme mods

[adamziel]

What changed

• Auto-merge independently inserted theme_mods_* option rows when both branches only add disjoint nav_menu_locations entries.
• Add a focused regression for the ID-banded wp_options.option_name collision shape that the runtime semantic E2E exposed.
• Add make test-cow-e2e-semantic, a shorter runtime E2E slice that starts a real ForkPress site, runs the WordPress semantic object graph merge, and exits before the long Git/crash/review tail.

Why

The new fast E2E slice reached a real gap: source and target branches can independently create theme_mods_twentytwentyfour rows with different ID-band option_id values but the same option_name. The merge previously treated that as a row unique collision before the existing serialized nav_menu_locations cell merge could apply.

Validation

• php -l scripts/cow/merge.php
• php -l tests/cow/merge.php
• git diff --check
• php tests/cow/merge.php passed, 2932 assertions
• make test-cow-semantic-fast passed
• make test-cow-e2e-semantic FORKPRESS_E2E_BIN=/tmp/forkpress-v0.1.27-linux-x86_64/forkpress reached the semantic merge and reproduced the pre-fix theme_mods_twentytwentyfour conflict in the released binary

Notes

I could not run the post-fix runtime E2E locally because this worktree has no dist/x86_64-unknown-linux-musl runtime bundle, and make forkpress stops until scripts/build-dist.sh has produced the static PHP runtime. CI/release artifact builds should exercise the new E2E target with a freshly embedded runtime.