fix(json): make JSON Schema enum compile again

[wolfy-j]

Problem

JSON Schemas containing enum fail during compilation in official builds:

json: cannot unmarshal into Go *interface {} within "/properties/state/enum/0": unsupported operation

kaptinlin/jsonschema uses the standalone go-json-experiment/json unmarshaler fallback contract. With GOEXPERIMENT=jsonv2, that module becomes an alias over the standard-library implementation, which uses a different, unexported skip sentinel. The dependency can no longer request default unmarshaling.

Resolution

• Explicitly build with GOEXPERIMENT=nojsonv2 so schema compilation uses the supported standalone implementation.
• Keep direct token-to-Lua decoding by importing github.com/go-json-experiment/json/jsontext directly.
• Replace the build-tagged v1/v2 split with one decoder, avoiding behavioral drift between build modes.
• Handle int64, larger JSON integers, floats, and unrepresentable numbers explicitly.
• Use the SHA-256 value directly as the schema-cache key instead of allocating a hex string.
• Cover enum compilation, invalid enum values, empty object identity, and numeric boundaries.

Performance

The fix does not retain the slower intermediate map[string]any decoder. Representative results on an AMD Ryzen 9 7950X3D, -benchtime=500ms, five runs:

Benchmark	Median	Bytes/op	Allocs/op
Decode simple	612 ns	1,017	18
Decode complex	2.49 µs	3,572	64
Decode large array	31.4 µs	55,394	721
Decode large dataset	1.37 ms	1,578,084	30,020
Decode nested config	11.7 µs	17,429	268

These allocation counts match the previous experimental fast decoder. Cached enum validation is approximately 2.2–2.9 µs at 2,443 bytes and 32 allocations per operation. Allocation profiling attributes the remaining validation cost primarily to kaptinlin/jsonschema evaluation-result construction.

Verification

• make lint
• make test-runtime
• JSON package race suite, 20 repetitions
• go mod tidy -diff
• Decode and cached enum-validation benchmarks