perf(json): presize maps during FromJson decode

[mizchi]

Summary

• Presize Map[String, V] from the input JSON object's known length.
• Avoid repeated growth and rehashing during FromJson decode.
• Add native release benchmarks for 10k and 100k entries.

Benchmark

Apple M5, MoonBit 0.1.20260824, native release; lower is better.

Entries	Before	After	Improvement
10,000	464 µs	261 µs	43.7%
100,000	5.76 ms	3.54 ms	38.5%

Command: moon bench --release --target native json/from_json_map_bench_test.mbt

Validation

• moon test json --target all: 215 passed on each backend
• moon check
• moon info; no public interface changes

[Copilot]

Pull request overview

Improve @json.FromJson decoding performance for Map[String, V] by pre-sizing the destination map based on the input JSON object size, and add targeted native-release benchmarks to track regressions/improvements.

Changes:

• Pre-size Map[String, V] in FromJson map decoding using the JSON object’s length.
• Add native benchmarks for decoding 10k and 100k-entry JSON objects into Map[String, Int].

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
json/from_json.mbt	Pre-sizes the decoded Map[String, V] to reduce growth/rehashing during decode.
json/from_json_map_bench_test.mbt	Adds native benchmarks for Map::from_json decode at 10k/100k entries.

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