Go 1.27 JSON compatibility tests

This module records observed semantic differences between Sonic and the JSON implementations available with Go 1.27. It does not change Sonic's production behavior.

See docs/sonic-go127-compatibility.md for the case matrix, interpretation rules, and release boundary.

Files

File	Responsibility
`catalog_test.go`	Sonic versus the active `encoding/json` backend
`jsonv2_test.go`	v1 facade versus experimental `encoding/json/v2` defaults
`backend_jsonv1_test.go`	Selects legacy v1 expectations
`backend_jsonv2_test.go`	Selects Go 1.27 default-backend expectations

All Sonic-versus-standard-library comparisons call sonic.ConfigStd directly. The only ConfigDefault calls are in tests that explicitly compare Sonic presets.

Run

# Go 1.27 default: encoding/json v1 API backed by jsonv2
env -u GOROOT GOTOOLCHAIN=go1.27rc2 \
  go test ./compatibility/ -count=1

# Go 1.27 legacy encoding/json backend
env -u GOROOT GOTOOLCHAIN=go1.27rc2 GOEXPERIMENT=nojsonv2 \
  go test ./compatibility/ -count=1

# Sonic optimized decoder, FastMap, and encoder VM
env -u GOROOT GOTOOLCHAIN=go1.27rc2 \
  SONIC_USE_OPTDEC=1 SONIC_USE_FASTMAP=1 SONIC_ENCODER_USE_VM=1 \
  go test ./compatibility/ -count=1

compatibility/ is a separate workspace module. A root-level go test ./... does not run it.