perf: consolidate filecache, timer calibration into performance module

Move filecache.zig and timer_fix.zig from standalone modules into
src/performance/. Filecache no longer has its own hooks/mutex — stats
are dumped by performance's worldupdate hook. Timer calibration runs
during performance install instead of transform44. Remove filecache
from build module list (enabled automatically with performance).
Merge DLL_README sections into single Performance entry.

build.zig

@@ -31,7 +31,6 @@ const module_list = [_]ModuleDesc{
     .{ .name = "dpslog", .desc = "Enable structured combat log events for addons", .default = false },
     .{ .name = "transform44", .desc = "Enable transform44 profiling/A/B testing (dev only)", .default = false },
     .{ .name = "addonperf", .desc = "Enable addon memory/CPU profiling API", .default = false },
-    .{ .name = "filecache", .desc = "Enable MPQ archive file cache" },
     .{ .name = "ssemaths", .desc = "Enable UnitXP x87 math polyfill replacements (SSE)", .default = false },
     .{ .name = "silicon", .desc = "Enable SSE2 math replacements (ported from libSiliconPatch)", .default = false },
     .{ .name = "performance", .desc = "Enable production SSE hooks (bone, particle, glyph cache)", .default = true },