* test(conformance): delta-format conformance suite (#262)

Check internal/delta against draft-fossil-delta-format-00, requirement by requirement: 39 requirements, 88 cases. Decoder cases (Apply and OutputSize) are byte vectors, each also given to fossil(1) through its SQL delta functions; fossil's verdict must match unless the case records why it follows the draft instead (fossil masks the high bit, does not verify the checksum, and returns the size as a signed int). Each rejection breaks one rule and keeps the trailer valid, so check order cannot hide the rule under test. Producer cases check Create against a strict oracle and against fossil's delta_apply, and fossil's delta_create against Apply.

FuzzDeltaApply compares Apply with an independent model decoder; FuzzDeltaRoundTrip checks Apply(Create(s, t)) == t. DF-3.1-6 (Create has no 2^32 bound) has no case: it needs a 4 GiB input.

* test(conformance): delta suite names the reason each rejection must give

A rejection case could pass because some other rule rejected the input. Every rejection case now names a substring of go-libfossil's error (or ErrChecksum), and the harness fails the case when the message differs.

Vectors that were rejected for the wrong reason are fixed or reattributed: copy-truncated (DF-4-2), reject-10-nul (non-empty count, so the NUL is the opcode), copy-overflow-64 (DF-4-11), and high-bit-count, which now tells a masking decoder from a conforming one (fossil accepts it). The phantom-base case uses a literal-only delta; the content-discard case checks ErrChecksum and the store-side header refusals move to DF-4.1-3. The allocation bound gets its own row (DF-6-5) and takes the least of five runs. DF-3.1-6 notes issue #279. Drops a duplicate size-query case, the sizeCase helper and the decorative fuzz guard bytes. check-in: 0629de2beb user: [email protected] tags: trunk