perf(table): decode metadata updates in one pass

[fallintoplace]

Summary

• Reuse one raw field map for update action dispatch and required-field validation.
• Keep legacy property field normalization working.
• Avoid repeated JSON object decoding for every update.
• Add canonical and legacy update benchmarks.

Benchmark

Apple M1 Pro, arm64. 1,000 updates:

Payload	Before	After	Result
Canonical	4.04 ms, 1.71 MB, 36,149 allocs	2.96 ms, 1.39 MB, 26,897 allocs	27% faster, 18% fewer bytes, 26% fewer allocs
Legacy properties	4.19 ms, 1.71 MB, 36,151 allocs	3.36 ms, 1.45 MB, 28,400 allocs	20% faster, 16% fewer bytes, 21% fewer allocs

Tests

• go test ./...
• go vet .