Enable go 1.27 support

[jprieto-temporal]

Adds support for Go 1.27. Follows the pattern of changes in #50.

[cockroach-teamcity]

This change is 

[RaduBerinde]

Thanks! Did you verify that the aliased structures in the runtime are unchanged (at least the prefix that we're using)?

[RaduBerinde]

Claude checked:

⏺ Verified — and I ran it, not just read it.

Source check (github.com/golang/go @ tag go1.27.0)

internal/abi.Type — field-for-field identical to swiss's rtType: Size_, PtrBytes uintptr; Hash uint32; TFlag, Align_, FieldAlign_, Kind_ uint8; Equal func; GCData
*byte; Str NameOff; PtrToThis TypeOff. (TFlag/NameOff/TypeOff are still uint8/int32/int32; Kind_'s declared type is now abi.Kind, which is uint8 — same size, no
layout effect.)

internal/abi.MapType (src/internal/abi/map.go) — the aliased prefix is unchanged:

type MapType struct {
Type
Key *Type
Elem *Type
Group *Type // swiss calls this Bucket — same position, same type
Hasher func(unsafe.Pointer, uintptr) uintptr
... // GroupSize, KeysOff, KeyStride, ElemsOff, ElemStride, ElemOff, Flags
}

Everything past Hasher was reworked in 1.27 for the mapsplitgroup experiment, but that's all beyond the prefix swiss reads. Note internal/abi/map_swiss.go and the
old OldMapType are gone — there's now just one MapType — so the "prefix of both abi.SwissMapType and abi.OldMapType" comment in swiss is stale.

runtime.eface — still {_type *_type; data unsafe.Pointer}.

runtime.fastrand64 — still exists as a push-linknamed compat shim (src/runtime/rand.go:341, //go:linkname legacy_fastrand64 runtime.fastrand64), so swiss's pull
linkname remains legal under the 1.23+ linkname restrictions.

Empirical check

Pulled the real go1.27.0 toolchain from the module proxy and ran swiss against it with -tags untested_go_version:

• Full suite: ok (plain, and -race -tags invariants).
• A layout test I added (/tmp/claude-503/swiss/runtime_layout_check_test.go) that cross-checks the aliased fields against reflect ground truth for 8 key types:
  rtEface.typ == the *abi.Type reflect reports for the map type; Size_/Align_/FieldAlign_/Kind_ match reflect.Type; Equal == nil, GCData != nil; mt.Key/mt.Elem are
  pointer-identical to typePtr(rt.Key())/typePtr(rt.Elem()) — which pins every offset up to and including Elem; Bucket/Group is a plausible non-nil type; Hasher is
  callable, deterministic, seed-sensitive, and well-distributed. All pass.
• Passes under both GOEXPERIMENT=mapsplitgroup and nomapsplitgroup.

Conclusion

Safe to widen the build tag to (go1.20 && !go1.28) || untested_go_version. This was tested on darwin/arm64 only, but since the struct definitions are textually
identical to swiss's copies, the layouts agree on every architecture. Worth also fixing the abi.SwissMapType/abi.OldMapType comment while you're in there.