perf(json): cache stringify indentation

[mizchi]

Cache pretty-print indentation strings by depth instead of rebuilding the same newline-and-spaces string for every array element or object member. Compact output keeps its existing branch and does not populate the cache.

Native benchmark (six nested arrays, 10,000 innermost values, indent=2):

before	after	change
1.53 ms	0.64 ms	58% faster

Validation: moon test json --target all, moon check --target all, and moon info.

[Copilot]

Pull request overview

Improves Json::stringify pretty-print performance by caching indentation strings per nesting depth, avoiding repeated construction of the same "\n" + spaces prefix during iterative stringification.

Changes:

• Replace per-write indentation string construction with a depth-indexed indent_cache and a write_indent helper.
• Add a new benchmark test for deep/wide JSON stringification with indent=2 to validate the optimization’s impact.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
json/json.mbt	Caches pretty-print indentation strings by depth and reuses them throughout Json::stringify when indent > 0.
json/stringify_indent_bench_test.mbt	Adds a benchmark covering deep nesting with many innermost elements to measure indentation caching wins.

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