fix(updater): stop re-downloading special-char mods

[Caedis]

Problem

Mods whose canonical filename contains characters stripped by SanitizeFilename (spaces, [], (), ') were written to disk under a sanitized name, but the scan filename index, persisted state.Mods[].Filename, and stale-jar matcher all keyed on the raw name. The on-disk file never matched, so every run re-downloaded the mod and orphaned the old jar.

Confirmed on a real instance: BiblioCraft[v1.11.7][MC1.7.10].jar -> BiblioCraftv1.11.7MC1.7.10.jar. 5 mods re-downloaded every run before the fix, 0 after.

Fix

• Record both names per mod: InstalledMod.RawFilename (canonical) and Filename (actual on-disk).
• Key the filename index on every variant via fileutil.FilenameVariants (raw, current sanitized, legacy sanitized) so a jar written by any past rule still resolves.
• reconcileSanitizedFilenames renames a drifted jar in place on a real run; skipped under --dry-run (no filesystem side effects).

Loosen sanitization

With the map bridging old/new names, SanitizeFilename now allows the actually-valid cross-OS set:

• keeps [] () ', spaces, + - . _
• decodes %XX (e.g. %2B -> +)
• replaces only < > : " / \ | ? * and control chars with _
• strips trailing dots/spaces, guards Windows reserved device names (CON/NUL/COM1-9/...)

Tests

• fileutil/sanitize_test.go — full rule table + FilenameVariants
• TestRun_SanitizedFilenameNotRedownloaded — legacy-named jar recognized as unchanged
• TestRun_LegacyNamedJarMigratedNoDuplicate — real update migrates in place, exactly one jar left
• TestReconcileSanitizedFilenames (+ skip stale/clean), updated assets/maven tests
• go build, go vet, go test ./... all pass

Closes #49
Refs #43