split: extract PRV_Settings_Controller from settings-page (300-line fix)

class-prv-settings-page.php was 346 lines — the only file over the
300-line cap. Extract the four POST handlers (handle_save,
handle_model_add, handle_model_update, handle_model_remove) into a
new PRV_Settings_Controller class (class-prv-settings-controller.php).

PRV_Settings_Page::register_hooks() instantiates the controller once,
passing a nonce-verification callback (bound to require_admin_nonce()).
All four admin_post_* hooks now delegate to the controller.

All phpcs:disable/enable WordPress.Security.NonceVerification.Missing
wrappers and sanitize_*/wp_unslash() calls traveled with the moved
handlers. PHPCS stays green.

Result:
  class-prv-settings-page.php       193 lines
  class-prv-settings-controller.php 213 lines
  All other production PHP files     ≤300 lines

Also: ARCHITECTURE.md file-tree updated; @see cross-refs added to
both class docblocks; CHANGELOG 0.2.0 CI section updated.

Version: 0.2.0

ARCHITECTURE.md

@@ -39,7 +39,8 @@ pr-vision/
 │   │   ├── class-prv-probe-runner.php        # Orchestrates run: lock + peptide*intent*model loop
 │   │   ├── class-prv-collector-registry.php  # Singleton registry for collectors + panels
 │   │   ├── class-prv-admin-page.php          # Dashboard page: menu, Run now, rendering
-│   │   ├── class-prv-settings-page.php       # [v0.2] Settings page: POST handlers + AJAX dispatch
+│   │   ├── class-prv-settings-page.php       # [v0.2] Settings page: menu, Run-now, AJAX dispatch; delegates POST to PRV_Settings_Controller
+│   │   ├── class-prv-settings-controller.php # [v0.2] POST handler impl: save config, model CRUD (split from settings-page)
 │   │   ├── class-prv-settings-renderer.php   # [v0.2] Settings HTML renderer (dark Assay theme)
 │   │   ├── class-prv-model-manager-table.php # [v0.2] Model manager table + Add form renderer
 │   │   └── class-prv-model-test-ajax.php     # [v0.2] Rate-limited AJAX handler for Test button