fix(songs): detect added/removed files on Reload Songs/Courses

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Why

With the default FastLoad=1, the song scan trusts the on-disk cache without verifying the song folder's directory hash. If a song folder is missing its music file at first scan, it gets cached with music_path: None, and the cache's path-existence check treats None as "present". So when the user later adds the music file and runs Reload Songs/Courses, the change is never detected. The song still won't preview or play until a folder is renamed (which changes the cache key) or the cache is wiped.

Approach

An explicit reload should always pick up on-disk changes, regardless of the FastLoad startup-speed toggle. So reload now forces cache freshness verification while startup stays fast:

• process_song takes a force_fresh flag and computes verify_freshness = force_fresh || !fastload. Cache reads stay fast; they are just verified against the directory hash.
• The flag is threaded through load_pack_scans, scan_and_load_songs_impl, and reload_song_dirs_impl.
• A new reload_all_songs_with_progress_counts (force-fresh) backs both full-reload call sites (options/reload.rs and select_music.rs); startup (init.rs) keeps the fast path unchanged.

The directory-hash check is symmetric, so this also catches file removals and delete-and-re-add with the same folder name.

Performance

This was measured, not just estimated. Microbenchmark over 2000 synthetic song folders (5 files each, warm cache, single-threaded, Windows):

cache read	per song
verify OFF (startup)	~263 us
verify ON (reload)	~706 us
delta	~443 us/song (~3x), ~0.44 s per 1000 songs

The extra cost is one canonicalize + read_dir + per-entry metadata per song. These are single-threaded numbers; the real reload parses in parallel, so wall-clock overhead is roughly delta / threads (~14 ms per 1000 songs on a 32-thread box). No extra chart parsing happens for unchanged songs, and startup is untouched. This is an acceptable trade for an occasional, progress-bar-driven action.

Notes for reviewers

• Startup behavior is intentionally unchanged: restarting the game alone will not pick up a missing-then-added file; detection is deferred to an explicit reload. (Removing a previously-present asset is still caught at startup via the existing path-existence check.)
• Two regression tests added in simfile/cache.rs cover the missing-then-added music case and a file-removal case.

Closes #493