v2.65.0 — Sprint 130: residual-literal pricing via greedy prepass

Extreme-mode ratio release; balanced/fast output unchanged, wire format
version 1 unchanged. Completes the literal-repricing arc: v2.64.0
priced literals from the raw block histogram, which is dominated by
exactly the repetitive content that matches remove — it underestimates
the entropy of the RESIDUAL literal stream the coder actually sees.

The optimal parser now runs a depth-4 greedy prepass on a private
throwaway matcher (accel on, ~1% of the DP's runtime, no shared-state
pollution), histograms the literal bytes of its token stream, and
prices literals from that residual distribution (still blended with
the flat prior; the blend re-swept to 6/8 — the honest histogram
tolerates a stronger weight than the raw one did, and the mode
contract now holds with 20 bytes of headroom on the tightest fixture
instead of violating). Falls back to the raw histogram when the
prepass cannot run.

Measured, extreme mode (corpus rev 2, zero roundtrip mismatches; full
tables in bench/COMPARISON.md): json 485,218 -> 451,744 (-6.9%; now
4.6% smaller than zstd-9's 473,364), xml -1.9% (7.3% under zstd-9),
csv -0.8%, logs -0.2%, plain text +1.6% and source +1.9% (both cells
still beat balanced and were already zstd-9 losses). Extreme now takes
two of the six text-family files from zstd-9 outright. Encode speed
unchanged (prepass ~1%).

Ratio-gate baseline regenerated (documented --update flow): json-mixed
and csv improve again; text-simple/varied/large give back 28/80/128
bytes; the known source-like contract violation narrows (849 -> 845).

Validation: 22/22 test suites under -O3 -flto and under
-fsanitize=address,undefined -fno-sanitize-recover=all; 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).