refactor(server): extract /api/tunings into routers/tunings.py + lib/appconfig.py (R3) (#858)

The merged-tuning-catalog route moves to lib/routers/tunings.py, verbatim except
@app->@router, CONFIG_DIR->appstate.config_dir, and the two seam substrates it
needed:

  - lib/appconfig.py — the pure config.json reader `_load_config` (used by ~11
    server sites + future config-reading routers). server.py re-imports it, so
    those call sites and any `server._load_config` test reference are unchanged.
  - appstate.tuning_providers — the TuningProviderRegistry instance injected by
    reference (a stable object mutated in place via register()/unregister()), so
    the router reads the same registry plugins populate through plugin_context.
    The instance stays defined in server.py, so `server.tuning_providers` still
    resolves — zero test retargets.

The tuning constants (DEFAULT_REFERENCE_PITCH, TUNING_PRESET_MIDIS,
freqs_to_midis) already live in lib/tunings.py and are imported directly.

server.py: 6,960 -> 6,917.

Verified: pyflakes clean (bar the pre-existing unused `tuning_name` import);
route table IDENTICAL (143); full pytest 2400 passed (110 tuning/config cases);
eslint 0. Boot smoke: /api/tunings serves referencePitch + tunings + tuningMidis.

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

lib/appconfig.py

@@ -0,0 +1,28 @@
+"""Reading the app's config.json — the one shared, pure helper (R3).
+
+Extracted verbatim from server.py so route modules that need a config value
+(reference pitch, server_config, …) can read it without reaching back into the
+host file. server.py re-imports it, so its ~11 call sites and any
+`server._load_config` test reference keep resolving unchanged.
+"""
+
+import json
+
+
+def _load_config(config_file):
+    """Read and parse config.json. Returns the parsed dict, or None if
+    the file is missing, unreadable, invalid JSON, or parses to a
+    non-dict (e.g. the file contains `[]` or `42`). Callers treat None
+    as "fall back to defaults". Shared between GET and POST so both
+    handle bad files the same way."""
+    if not config_file.exists():
+        return None
+    try:
+        # Explicit UTF-8: save_settings()/import write config.json as
+        # UTF-8 bytes, so the read must not depend on the platform's
+        # default text encoding (cp1252 on Windows would mojibake or
+        # UnicodeDecodeError on a non-ASCII DLC path).
+        parsed = json.loads(config_file.read_text(encoding="utf-8"))
+    except Exception:
+        return None
+    return parsed if isinstance(parsed, dict) else None

lib/appstate.py

@@ -61,6 +61,10 @@ copies a hardcoded file list — that regression is what moved this file here.
 # The singletons routers may read. Every name here must also be a `_SLOTS` key.
 meta_db = None
 audio_effect_mappings = None
+# The tuning-provider registry instance (built-ins + plugin-contributed). A
+# stable object mutated in place via register()/unregister() — injected here by
+# reference so routers read the same registry plugins populate.
+tuning_providers = None
 
 # Config paths. server.py derives these from the environment (fresh on every
 # import, so the ~49 pop-and-reimport fixtures keep working) and injects them
@@ -90,7 +94,7 @@ builtin_diagnostic_filename = None
 running_version = None
 
 _SLOTS = frozenset({
-    "meta_db", "audio_effect_mappings",
+    "meta_db", "audio_effect_mappings", "tuning_providers",
     "config_dir", "dlc_dir", "dlc_dir_env",
     "static_dir", "sloppak_cache_dir", "audio_cache_dir",
     "get_progression_content", "builtin_diagnostic_filename",

lib/routers/tunings.py

@@ -0,0 +1,46 @@
+"""The merged tuning catalog (/api/tunings).
+
+Extracted verbatim from server.py (R3) except @app->@router, CONFIG_DIR->
+appstate.config_dir, _load_config imported from lib/appconfig, and the tuning
+registry read through the appstate seam (appstate.tuning_providers — the same
+instance plugins register into via the plugin_context in server.py).
+"""
+
+from fastapi import APIRouter
+
+import appstate
+from appconfig import _load_config
+from tunings import DEFAULT_REFERENCE_PITCH, TUNING_PRESET_MIDIS, freqs_to_midis
+
+router = APIRouter()
+
+
+@router.get("/api/tunings")
+def get_tunings():
+    cfg = _load_config(appstate.config_dir / "config.json") or {}
+    ref = cfg.get("reference_pitch", DEFAULT_REFERENCE_PITCH)
+    try:
+        ref = float(ref)
+        if not (430.0 <= ref <= 450.0):
+            ref = DEFAULT_REFERENCE_PITCH
+    except (TypeError, ValueError):
+        ref = DEFAULT_REFERENCE_PITCH
+    merged = appstate.tuning_providers.get_merged(ref)
+    # tuningMidis: the same catalog as exact integer MIDI notes (low → high).
+    # Built-ins come straight from TUNING_PRESET_MIDIS (no float round-trip);
+    # provider-contributed entries are recovered from their frequencies at the
+    # served reference pitch. Every consumer today (the v3 badges, plugins)
+    # reconstructs midis client-side via log2 — a rounding footgun at non-440
+    # references — so serve the integers once, host-side. Additive: the
+    # existing referencePitch/tunings shape is unchanged.
+    tuning_midis: dict[str, dict[str, list[int]]] = {}
+    for key, names in merged.items():
+        builtin = TUNING_PRESET_MIDIS.get(key, {})
+        resolved: dict[str, list[int]] = {}
+        for name, freqs in names.items():
+            midis = builtin.get(name) or freqs_to_midis(freqs, ref)
+            if midis:
+                resolved[name] = list(midis)
+        if resolved:
+            tuning_midis[key] = resolved
+    return {"referencePitch": ref, "tunings": merged, "tuningMidis": tuning_midis}