Accept feedpak name/extension as additive alias for sloppak

Phase 1 of deprecating the format name sloppak -> feedpak. Purely additive
and fully back-compat: nothing sloppak stops working, and no on-disk/runtime
artifacts (cache dir, config, SLOPSMITH_* env) are touched.

- lib/sloppak.py: detection accepts both `.feedpak` and `.sloppak`
  (new PACK_SUFFIXES / is_pack / is_feedpak; is_sloppak kept as a deprecated
  alias that now matches both). Read the optional top-level `feedpak_version`
  manifest key onto LoadedSloppak and into extract_meta(). Add LoadedFeedpak
  class alias and a deprecation note in the module docstring.
- lib/feedpak.py (new): canonical module name; re-exports the sloppak public
  API unchanged so new code can `import feedpak`.
- server.py: accept `.feedpak` uploads (_ALLOWED_SONG_EXTS + message); add a
  `get_feedpak_cache_dir` plugin-context key alongside the legacy
  `get_sloppak_cache_dir` (same dir); tolerate a `feedpak` value in the
  library format filter.
- static/: accept `.feedpak` in the upload picker (index.html accept=, app.js
  client-side filter).
- tests/test_feedpak_alias.py: both extensions detected + loaded via both
  module names, feedpak_version read, legacy aliases intact.

The emitted `format` library tag stays "sloppak" for both extensions this
phase (flipping it to "feedpak" with badge/gating updates is a deliberate
frontend-touching follow-up). Format spec: got-feedback/feedback-feedpak-spec.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Signed-off-by: Kris Anderson <topkoa@gmail.com>

lib/feedpak.py

@@ -0,0 +1,50 @@
+"""feedpak — open song-package format loader (canonical module name).
+
+The format was renamed `sloppak` → `feedpak`. The implementation currently still
+lives in :mod:`sloppak`; this module is the **canonical name going forward** and
+re-exports that public API unchanged, so new code can do::
+
+    import feedpak
+    loaded = feedpak.load_song(filename, dlc_root, cache_root)
+
+Both `.feedpak` and `.sloppak` packs are accepted everywhere (the legacy
+extension and the `sloppak` module name are kept as permanent deprecated
+aliases). The authoritative on-disk format spec is published at
+https://github.com/got-feedback/feedback-feedpak-spec.
+
+When the internal rename lands, the implementation can move into this module and
+`sloppak.py` becomes the thin re-export shim instead — importers of `feedpak`
+will not need to change.
+"""
+
+from __future__ import annotations
+
+from sloppak import (  # noqa: F401  (re-export)
+    PACK_SUFFIXES,
+    LoadedFeedpak,
+    LoadedSloppak,
+    extract_meta,
+    get_cached_source_dir,
+    is_feedpak,
+    is_pack,
+    is_sloppak,
+    load_manifest,
+    load_song,
+    read_feedpak_version,
+    resolve_source_dir,
+)
+
+__all__ = [
+    "PACK_SUFFIXES",
+    "LoadedFeedpak",
+    "LoadedSloppak",
+    "extract_meta",
+    "get_cached_source_dir",
+    "is_feedpak",
+    "is_pack",
+    "is_sloppak",
+    "load_manifest",
+    "load_song",
+    "read_feedpak_version",
+    "resolve_source_dir",
+]