refactor!: ASAPv1 as the only serialization — remove asapmsgpack and gob

[GordonYuanyc]

Stacked on #95 (asapv1-only/hydra). Final PR of the sketchlib-go ASAPv1-only series: removes the last non-ASAPv1 serialization. With the stack (#85 → #86–#93 via the integration branch → #94 → #95 → this) ASAPv1 is the only format in sketchlib-go; proto/ and the protobuf harness were already removed in #95.

Removed

• wire/asapmsgpack/ (encoder, decoder, cell-delta; no importers left).
• The gob format: common.EncodeToBytes / DecodeFromBytes and every SerializeToBytes / Deserialize*FromBytes on KLL, HLL (three types), Count-Min, Count Sketch, FoldCountMinSketch, FoldCountSketch and Vector2D; SpaceSaving's GobEncode / GobDecode; the helpers only the gob decoders used.
• Vector1D's JSON codec (only its own test used it).
• Stale .gitignore lines for the deleted cross-language harness.

Docs

README gains a short "Serialization" section: ASAPv1 via wire/asapv1 and per-sketch MarshalASAPv1 / UnmarshalASAPv1, the spec in asap_sketchlib, and the golden repo mounted at asapv1_golden/.

Open question

These Go types now have no serialized form: FoldCountMinSketch, FoldCountSketch (gob was their only format; ASAPv1 kinds 0x1b/0x1c are reserved, not designed), SpaceSaving (its gob stored only k), CountL2HH, ExponentialHistogram, NitroSketch, OctoSketch. Rust already implements SpaceSaving (0x18), CountL2HH (0x19) and ExponentialHistogram / EHSketchList (0x13/0x14); Go codecs for those could follow.

Verification (Go 1.24.9)

go build ./..., go vet ./..., go test ./... (all passing on this run; TestKLL_Reset_SubsequentInserts is a known flake), go mod tidy -diff clean; gofmt -l lists only two files already unformatted on main. Implemented and reviewed by separate agents; review findings are addressed in the last commit. Rebased onto #95's review fixes.

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