generator: cap block s2length at the negotiated checksum length#961
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sum_sizes_sqroot() capped the strong-sum length at SUM_LENGTH (16), the legacy MD4/MD5 digest size. Since 0902b52 the sum2 array elements are xfer_sum_len bytes and the sender rejects a sums header whose s2length exceeds xfer_sum_len. When the negotiated transfer checksum is shorter than 16 bytes -- xxh64 (8), used when the build's libxxhash lacks xxh128/xxh3 (e.g. Ubuntu 20.04) -- the generator still emitted s2length up to 16, so --append-verify and other full-checksum (redo) transfers died with "Invalid checksum length 16 [sender]" (protocol incompatibility). Cap s2length at MIN(SUM_LENGTH, xfer_sum_len): unchanged for any checksum >= 16 bytes (md5/xxh128/sha1), corrected for short ones. Also closes a latent over-read of the xfer_sum_len-sized digest buffer.
Forces --checksum-choice=xxh64 (an 8-byte transfer checksum) with a corrupted-prefix --append-verify so the full-checksum redo path runs. Before the generator capped s2length at MIN(SUM_LENGTH, xfer_sum_len) this died with "Invalid checksum length 16 [sender]"; the test is RED on the prior generator and GREEN with the cap. Reproduces on any build that has xxhash, so it guards the fix without an old-libxxhash host; skips when xxh64 is absent (a build without xxhash).
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fixes a hash issue on old hosts (tested on ubuntu 20.04) with old libxxhash.