chaining: the `v[]` peak-score recurrence in `mb_lchain_dp` is dead

[nh13]

v[] is allocated at lchain.c:179 and filled by the peak-score recurrence at lchain.c:217:

v[i] = max_j >= 0 && v[max_j] > max_f? v[max_j] : max_f; // v[] keeps the peak score up to i

Those values are never consumed. v[max_j] on that same line is the only reader, so the recurrence feeds nothing but itself. Inside mb_chain_backtrack v is write-only (v[n_v++] = i, lchain.c:81), and compact_a reads the anchor indices written there (lchain.c:106), never the DP scores.

The array itself must stay — the backtrack uses it as n-element scratch — but the recurrence and the int32_t it computes per anchor are dead.

Verification

Deleted line 217 outright, rebuilt, and diffed SAM against stock 95a79d4 (r419). All runs exited 0 with the expected record counts, and all four outputs were byte-identical:

dataset	records	result
chrM PE (test/)	2,016 lines	identical
HG002 HiFi, 2k reads vs hg38	5,590 lines	identical
HG002 ONT R10, 2k reads vs hg38	6,117 lines	identical
HG002 WGS, 200k pairs vs hg38	405,287 lines	identical

Also inherited from minimap2 (lchain.c:202), where the same line appears in the RMQ variant at :354 — worth checking whether it is live there before assuming it is dead everywhere.

Expected impact

Negligible as a speedup — one compare and one store per anchor in the outer loop of a DP whose inner loop runs up to max_iter times per anchor. Not worth doing for performance. Worth doing so the next reader does not have to work out that the comment is describing a value nobody uses.