refactor(server): extract the diagnostics routes into routers/diagnostics.py (R3) (#857)

The three /api/diagnostics/* routes (export, preview, hardware) plus their
exclusive payload-cap helpers and the `_diag_*` normalisers. Bodies verbatim
except @app->@router, CONFIG_DIR->appstate.config_dir, _running_version()->
appstate.running_version() (a new seam slot; the impl stays in server.py where
the settings region also calls it), and the builtin-plugins lookup in
_diag_plugins_roots: Path(__file__).parent -> Path(__file__).resolve().parents[2]
(routers -> lib -> app root; plugins/ ships at the app root in every packaging
path).

The pure caps/normalisers (_diag_cap_console/_dict/_contributions,
_diag_coerce_bool, _diag_normalize_include, _DIAG_MAX_*) are re-exported from
server.py so the existing `server._diag_*` / `server._DIAG_*` tests keep
resolving — none of them monkeypatch these, so no test retargets.

server.py: 7,216 -> 6,960.

Verified: pyflakes clean (bar the intentional re-export lines); route table
IDENTICAL (143); full pytest 2399 passed (77 diag/packaging + 122 diagnostic-
matched cases incl the cap/coerce/normalize suites); eslint 0; Codex pending.
Boot smoke: /hardware, /preview, and POST /export (200 application/zip) all
serve from the new router location.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/appstate.py

@@ -87,12 +87,14 @@ audio_cache_dir = None
 # in server.py (its `setattr(server, "_progression_content")` test is untouched).
 get_progression_content = None
 builtin_diagnostic_filename = None
+running_version = None
 
 _SLOTS = frozenset({
     "meta_db", "audio_effect_mappings",
     "config_dir", "dlc_dir", "dlc_dir_env",
     "static_dir", "sloppak_cache_dir", "audio_cache_dir",
     "get_progression_content", "builtin_diagnostic_filename",
+    "running_version",
 })