Document the per-assertion parsing cost

Each assertion builds its own parser and walks the whole class set, so the
cost is linear in the number of rules: measured on 619 files, one pass takes
~2s and ten rules take ~20s, while the CLI parses once and checks every rule
against the result.

Only document it here. The fix is a memoizing Parser decorator, and that
implements a core interface over core types, so it belongs in
phparkitect/phparkitect next to FileParser, where the CLI and any other
integration get it too.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FE8ESEV7TzGLDwJuTfdq95

Author: claude

README.md

@@ -65,6 +65,18 @@ reference:
 Anything you can express in a `phparkitect.php` config file works here unchanged. Baselines and the
 other CLI-only options do not — for those, keep using `vendor/bin/phparkitect check`.
 
+## Performance
+
+Every assertion parses the class set from scratch, so the cost grows linearly with the number of
+rules: one pass over ~600 files takes about 2 seconds, so ten rules cost about 20. The CLI does not
+work this way — it parses each file once and then checks every rule against it, which makes extra
+rules essentially free.
+
+Keep your class sets narrow (`ClassSet::fromDir(__DIR__.'/../src/Domain')` rather than the whole
+`src/`), and reach for the CLI when you have many rules over a large codebase. Removing this
+difference means teaching the core analyzer to reuse parsed files across runs, which belongs in
+`phparkitect/phparkitect` rather than here.
+
 ## Compatibility
 
 PHP 8.0–8.5, PHPUnit 9.6/10/11/12, PHPArkitect `^1.0`. Every combination is exercised in CI.