refactor(coco)!: ASAPv1 only — drop the proto format, add a golden

[GordonYuanyc]

Part of the ASAPv1-only series: one PR per sketch group, each off main. This one removes CocoSketch's protobuf format; ASAPv1 (0x0c 0x00) is the only Coco format left, and it gets its first golden.

Removed

• proto/cocosketch/cocosketch.proto (CocoSketchState) and its tools/gen-proto entry; SketchEnvelope.coco (17) is now reserved 17; reserved "coco";.
• The Coco section of tests/xtest_consumer.rs and its helpers. Coco had no portable or native shim.

Added

• Golden coco_3x7 (pushed to sketchlib-golden-bytes; submodule bumped): 15 of 21 buckets set directly; nil buckets, the empty key, a 31-byte fixstr / 32-byte str8 key, 2-, 3- and 4-byte UTF-8 keys, every uint width up to u64::MAX, and an occupied bucket holding 0. Checked in both directions.
  • The width is 7, not a power of two, so the fixture pins the spec's % cols fold: 11 of the 15 keys sit in odd columns, which a mask fold cannot produce (verified by swapping the fold to a mask and watching the test fail).
  • Coco's decoder checks each key's placement against its hash (spec §3.12), so this is the one fixture whose decode depends on the hash function; the golden README says so.
• The ASAPv1 docs no longer state a fixture count; they defer to the golden README.

Notes

• Coco's RNG seed and state stay out of the payload.
• For sketchlib-go: decoding this golden requires Go's Coco to store full keys and use per-row seed_list[r] with % cols instead of one seed with a mask, i.e. an algorithm change, not just a codec.
• Out of scope, existing code: coco.rs / coco/wire.rs compute hash as usize % cols, which differs from the spec's 64-bit % cols on 32-bit targets when cols is not a power of two.

Verification (rustc 1.99)

cargo fmt --check; clippy -D warnings for all four CI feature sets; RUSTDOCFLAGS="-D warnings" cargo doc; gen-proto drift clean; asapv1_golden, --lib -- coco, pbt -- coco, xtest_consumer. The reviewer hand-decoded the golden and re-checked placement with an independent xxh3 program. Implemented and reviewed by separate agents; review findings are addressed in the third commit.

🤖 Generated with Claude Code

[GordonYuanyc]

Outdated. ASAPv1 encodings and goldens landed in #232 and #235 against the current spec; removing the legacy formats is redone on current main in #206 and #207.