Add read-ahead buffering for small random-access reads

[neeraj-pilot]

Adds an opt-in ReadAheadRASource for consumers that perform many small, nearby reads and seeks. Across all eight exif_reader fixtures, it reduced median warm-cache macOS AOT latency from 97.2 ms to 7.0 ms and underlying FileRASource read calls from 4,974 to 19, with exact EXIF output parity.

Fixture	Baseline	Read-ahead	Speedup	Read calls
AVIF	6.94 ms	0.36 ms	19.4×	381 → 2
HEIC	59.88 ms	2.03 ms	29.6×	3,230 → 3
JXL	0.98 ms	0.13 ms	7.5×	43 → 2
JXL/Brotli	0.81 ms	0.50 ms	1.6×	12 → 3
PNG	1.95 ms	0.14 ms	13.9×	103 → 1
CR3	1.84 ms	1.35 ms	1.37×	19 → 3
RAF	17.86 ms	2.23 ms	8.0×	862 → 3
WebP	6.12 ms	0.29 ms	20.9×	324 → 2

[mgenware]

Thank you! Published 4.3.0