fix: Board admin submenu 404 — register after parent menu (v0.19.1)

Root cause: PRAutoBlogger_Board_Page::on_register_menu was hooked at
admin_menu default priority 10, same as PRAutoBlogger_Admin_Page::on_register_menu,
but registered first in register_admin_hooks(). WordPress calls
get_plugin_page_hookname() at add_submenu_page() time; if add_menu_page()
has not yet fired, $admin_page_hooks['prautoblogger-settings'] is unset and
WP falls back to the admin_page_prautoblogger-board hookname. At request
time WP recomputes the hookname as prautoblogger_page_prautoblogger-board,
finds no registered callback, and calls wp_die() — 404. Same mismatch
prevented board.css and board.js from loading (enqueue gate also checks
prautoblogger_page_ prefix).

Fix:
- class-prautoblogger.php: parent menu (Admin_Page) stays at priority 10;
  board submenu (Board_Page) moves to priority 11. Explicit priorities added
  with explanatory comment. See ARCHITECTURE.md §Board for the constraint.
- class-board-page.php: add $submenu reorder in on_register_menu() to keep
  Board as the first submenu item (primary click target), since the board
  entry can no longer be prepended by the auto-duplicate mechanism.
- tests/unit/Admin/BoardMenuRegistrationTest.php: 4 tests (new file):
  (1) priority relationship regression guard — FAILS on unfixed code (both
      at 10), PASSES with fix (board at 11); (2) fixed execution order;
      (3) broken execution order documentation; (4) parent slug correctness.
- ARCHITECTURE.md: §22b updated with menu-ordering constraint.
- CHANGELOG.md: v0.19.1 entry.

Deletion audit: only intentional removals (old wrong-order registration block).
class-prautoblogger.php is 407 lines (was 400 pre-fix, pre-existing P2 debt).

Agent-Session: board-404-hotfix-20260612

CHANGELOG.md

@@ -5,6 +5,26 @@ All notable changes to PRAutoBlogger will be documented in this file.
 Format follows [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project uses [Semantic Versioning](https://semver.org/).
 
+## [0.19.1] - 2026-06-12
+
+### Fixed
+- **Board admin submenu 404 (P0 hotfix).** `PRAutoBlogger_Board_Page::on_register_menu`
+  was hooked at `admin_menu` priority 10, identical to
+  `PRAutoBlogger_Admin_Page::on_register_menu`. Because the board hook was
+  registered first in `register_admin_hooks()`, `add_submenu_page()` fired while
+  `$admin_page_hooks['prautoblogger-settings']` was still unset. WordPress fell back
+  to the `admin_page_prautoblogger-board` hookname; at request time it recomputed
+  `prautoblogger_page_prautoblogger-board`, found no registered callback, and called
+  `wp_die()` with "Invalid plugin page". Board CSS/JS also never loaded (same
+  enqueue-gate mismatch). Fix: parent menu hook stays at priority 10; board submenu
+  hook moves to priority 11, guaranteeing `add_menu_page()` fires first. Board is
+  kept as the first submenu item via explicit `$submenu` reorder in
+  `PRAutoBlogger_Board_Page::on_register_menu()`.
+- **Regression test added:** `tests/unit/Admin/BoardMenuRegistrationTest.php` —
+  three assertions covering priority relationship, execution order, and parent-slug
+  correctness. Includes a "documents the bug" test that fires the unfixed hook order
+  to prove the broken state.
+
 ## [0.19.0] - 2026-06-12
 
 ### Added