feat(logger): add PSR-3 logger that captures wpseo_logger output

[diedexx]

Summary

This PR can be summarized in the following changelog entry:

• Adds a Logger panel to the Yoast Test Helper (Tools → Yoast Test) that replaces Yoast SEO's NullLogger via the wpseo_logger filter and persists captured entries to either a custom DB table or a JSONL file.

Relevant technical choices

• No new Composer dependency. The logger extends \YoastSEO_Vendor\Psr\Log\AbstractLogger (Yoast SEO's prefixed copy) so the resulting object satisfies the same LoggerInterface Yoast SEO's filter is typed against. Filter registration is gated by class_exists('\YoastSEO_Vendor\Psr\Log\AbstractLogger') so the test helper does not fatal when Yoast SEO is inactive.
• Two backends, picked via dropdown: a dedicated {prefix}yoast_test_helper_log table created lazily via dbDelta on first use (capped at 1000 rows; cap is filterable through Yoast\WP\Test_Helper\logger_max_rows), or a JSONL file at a user-configurable absolute path (capped at 5MB with rotation by halving). The DB backend uses %i identifier placeholders so $wpdb->prepare() handles table-name escaping.
• File-write concurrency: write(), clear(), and rotation all happen under a single flock(LOCK_EX) per call, so concurrent writers don't lose data during rotation.
• Path validation rejects empty, non-absolute, ..-containing, non-writable, and paths inside WP_CONTENT_DIR / the uploads directory — those are typically web-accessible. UI also shows an inline warning under the path field.
• Configurable level threshold (debug → emergency); entries below are dropped at capture time. PSR-3 §1.2 placeholder interpolation, with non-scalar context values JSON-encoded so {date}/{list}/{object} substitute meaningfully.
• Viewer: compact summary with a native HTML <dialog popover> for the full table — no JavaScript required for open/close/ESC. Capped at 1000 entries; renders "1000+ logs captured" when there's more behind the cap. View / Delete are link-style buttons (Delete is button-link-delete, red), keeping Save as the only primary CTA.
• Backend toggle JS (the only JS in this PR) hides the file-only fields when the database backend is selected.

Milestone

• I've attached the next release's milestone to this pull request.

Test instructions

Test instructions for the acceptance test before the PR gets merged

This PR can be acceptance tested by following these steps:

1. Activate Yoast SEO and the Yoast Test Helper.
2. Visit Tools → Yoast Test. Confirm a new Logger block appears.
3. Database backend: enable the logger, leave backend = Database, level = Debug, save. Confirm the table exists: wp db query "SHOW TABLES LIKE '%yoast_test_helper_log'".
4. Trigger a log call, e.g. wp eval 'apply_filters("wpseo_logger", new \YoastSEO_Vendor\Psr\Log\NullLogger())->warning("hello {who}", ["who" => "world"]);'.
5. Reload the panel; confirm "1 log captured." Click View captured logs — the popover shows the entry. Close it. Click Delete all captured logs — the table empties.
6. File backend: switch backend = File, set path to /tmp/yoast-test-helper.log, save. Repeat the wp eval from step 4. tail /tmp/yoast-test-helper.log shows one JSON line containing "level":"warning" and "message":"hello world". Reload panel, confirm the viewer shows the entry. Click Delete — file truncates to 0 bytes.
7. Threshold: set level = Error, fire a ->warning(...) and a ->critical(...). Only the critical entry persists.
8. Invalid path: backend = File, path = /wp-content/uploads/anything.log. Confirm an error notification renders, the path doesn't save, and the logger doesn't switch on.
9. Backend toggle: set backend = Database. Confirm the file path field and warning hide. Switch back to File. Confirm they reappear.
10. Run composer check-cs-thresholds; should pass at the existing baseline (32 errors, 0 warnings).

Test instructions for QA when the code is in the RC

• QA should use the same steps as above.

Fixes #87