v2.65.2 — Sprint 132: extreme-mode encode ~2x faster, byte-identical

Pure speedup of the extreme-mode optimal parser; output is
byte-identical to v2.65.0/1 (verified per file and by the ratio gate
at ±0), wire format unchanged.

Two changes in the candidate collector, both outcome-preserving:

• Rep-offset probes now extend through extend_match (8-byte xor/ctz
  stride) instead of a byte-at-a-time loop. The probe runs up to three
  times at every DP position.
• The hash-chain walk exits as soon as a LONG_MATCH-class candidate
  (>= 512) appears. The DP takes such a candidate immediately and
  ignores all others, so the remainder of the depth-256 walk — with an
  extend per prefix hit — was pure waste on repetitive regions.

Measured extreme encode (CLI, corpus rev 2): logs 1.1 -> 2.0 MB/s,
json 1.2 -> 2.2, xml 1.2 -> 2.2, text 4.0 -> 5.2, source 2.9 -> 4.3,
csv 1.3 -> 1.5. Lowering the LONG_MATCH threshold itself (256/128) was
also swept: size-neutral within ±130 bytes and only marginally faster,
so the threshold stays at 512 and the release stays byte-identical.

Validation: 22/22 test suites under -O3 -flto and under
-fsanitize=address,undefined -fno-sanitize-recover=all; ratio gate
±0 bytes (output identity); 5,200 differential fuzz cases consistent;
27/27 negative-corpus cases; ASan+UBSan+LeakSanitizer roundtrip sweep
clean (11 corpus files x 3 modes, byte-exact).