[codex] Expose branch preview PHP fatals

[adamziel]

What it does

Makes branch-preview PHP fatals visible in ForkPress logs instead of leaving users with only WordPress' generic critical-error screen.

The change:

• disables WordPress' fatal-error recovery handler in generated COW/CAS branch configs
• adds a COW router shutdown logger that records branch name, request URI, file, line, and fatal message to wp-debug.log
• preserves that setting when Git pushes rewrite branch wp-config.php
• documents that branch-preview fatals should now be visible through forkpress logs --file wp

Rationale

Remote clones can boot far enough for WordPress to load plugins/themes, then fatal inside a site-specific plugin. WordPress recovery mode masks that as "There has been a critical error on this website", and the current logs can lack the branch/request context needed to debug the cloned site.

Implementation

runtime/cow/router.php now registers a shutdown handler before requiring branch PHP. On fatal shutdown it writes a structured line to the ForkPress debug log and PHP error log. Generated configs define WP_DISABLE_FATAL_ERROR_HANDLER so PHP fatals flow to ForkPress logging instead of WordPress' generic recovery page. Git-side wp-config.php normalization inserts or normalizes the same constant when pushed configs are applied to materialized branches.

Testing instructions

Checked locally:

php -l runtime/cow/router.php
php -l runtime/cow/bootstrap_wp.php
php -l scripts/cow/git_server.php
php -l tests/cow/router_paths.php
php -l tests/cow/git_server.php
php tests/cow/router_paths.php
php tests/cow/git_server.php
FORKPRESS_RUNTIME_BUNDLE=/dev/null cargo test -p forkpress-cli remote_clone
cargo fmt --check
git diff --check