perf(rest): cache partition decode plans

[fallintoplace]

Summary

• Cache the resolved partition decode plan by spec ID while decoding REST scan tasks.
• Reuse the plan across inline and fetched task envelopes in one remote scan.
• Keep decoded partition values and maps independent for every file.
• Add focused reuse tests and a decoder benchmark.

Performance

Compared with the parent commit on an Apple M1 Pro, using GOMAXPROCS=1 and 5 benchmark samples:

• 64 tasks: 365 us -> 241 us, about 1.5x faster
• 1024 tasks: 4.23 ms -> 3.82 ms, about 1.1x faster
• Allocations: 2,049 -> 1,862 per operation, about 9% fewer

The cache is shared across inline and fetched task envelopes, so the setup cost is paid once per spec for a remote scan.

Testing

• go test ./... -count=1
• go test -race ./catalog/rest -count=1
• go vet ./catalog/rest
• go test ./catalog/rest -run "^$" -bench "^BenchmarkDecodeScanTasks$" -benchmem -benchtime=300ms -count=3