Add opt-in slice presize (//lightning:presize directive + ,presize tag)

[JohanLindvall]

What

Adds an opt-in presize for slices of leaf-collection structs: preallocate a slice to its counted element length (one make) instead of letting it grow by append-doubling. Enabled two ways:

• //lightning:presize — type directive, applies to every eligible slice the root reaches.
• ,presize — json-tag option, per-field; propagates through slices/maps/pointers and stops at struct boundaries, exactly like nocopy/lax.

//lightning:presize
type Doc struct { ... }          // whole-type

type Doc struct {                // or surgically, per field
    Areas []struct{...} `json:"areas,presize"`
}

Why off by default

It's a space-for-time trade. Measured with ,presize on citm's areas field:

metric	vs base
allocs/op	−54%
B/op	−46%
time	+5.6…7.6%

The CountArrayElements scan doubles the per-element walk, which costs more wall-time than the reallocations it saves on tiny (~30-byte) elements — so on the time-focused microbenchmarks it's a small loss. It's a win on time too when arrays are large (marine_ik bones[].pos: −3.8% time, −31% B/op). The split is array/element-size dependent and not statically decidable, so it's an explicit opt-in rather than a cost heuristic — aimed at long-running services that care about GC pressure.

Safety

structSkipIsCheap gains a relaxed arg (gated by isLeafType) that admits structs whose only non-flat fields are 1-D collections of scalars/strings/any (citm areas[].blockIds is a []any that's always []). It still rejects collection-of-collection and collection-of-struct, so the multi-dimensional coordinate arrays slicePresize deliberately skips (the documented +155% disaster on canada/large-json) are never presized regardless of the directive. The flag threads through field/valueDecoder/sliceDecoder/arrayDecoder/mapDecoder with a Presize suffix/memo-key bit so tagged and untagged slices of the same type get distinct decoders.

Tests

• TestPresizeDirective — directive presizes leaf-collection slices, leaves multi-dim arrays correct, handles empty/non-empty/null.
• TestPresizeTag — tagged vs untagged slices decode identically.
• Full suite green; generator runs clean over the whole bench corpus; gofmt clean.

CLAUDE.md updated (directive list, slicePresize opt-in entry with numbers + rationale, cross-ref from the "tried and rejected" entry).

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