Optimize ASCII-safe string rendering

[He-Pin]

Summary

• Optimize rendering for strings that are already proven ASCII-safe.
• JVM/Native use String#getBytes(srcBegin, srcEnd, dst, dstBegin) to copy low bytes directly into the renderer buffer without allocation.
• Mark ASCII-safe std.base64Decode results so decoded ASCII text can use the same renderer fast path.

Motivation

The docs base64 benchmarks fold into long ASCII-safe strings, then spend visible time rendering those strings. This keeps the same ASCII-safe precondition but moves JVM/Native to a platform copy path that is friendlier to JIT and GC, and extends the same tagging to decoded base64 text when the decoded bytes are JSON-safe ASCII.

Benchmarks

Baseline is upstream/master at 8b67cb1e.

JMH, docs regression cases:

case	upstream	this PR	delta
go_suite/base64.jsonnet	0.168 ms/op	0.109 ms/op	35.1% faster
go_suite/base64Decode.jsonnet	0.124 ms/op	0.076 ms/op	38.7% faster
go_suite/base64_byte_array.jsonnet	0.915 ms/op	0.782 ms/op	14.5% faster

Native CLI hyperfine against latest source-built jrsonnet master 5b43fa8 (jrsonnet 0.5.0-pre98):

case	upstream	this PR	latest jrsonnet	highlight
go_suite/base64.jsonnet	5.3 +/- 2.2 ms	4.8 +/- 0.4 ms	2.7 +/- 0.6 ms	this PR narrows the gap from 1.94x to 1.77x slower; CLI timings are short/noisy

Validation

• git diff --check
• ./mill -i __.checkFormat
• ./mill -i 'sjsonnet.jvm[3.3.7].test'
• ./mill -i 'sjsonnet.js[3.3.7].test'
• ./mill -i 'sjsonnet.native[3.3.7].test'
• ./mill -i 'sjsonnet.jvm[2.12.21].compile' 'sjsonnet.jvm[2.13.18].compile' 'sjsonnet.native[2.13.18].compile' 'sjsonnet.js[2.13.18].compile'
• ./mill -i 'sjsonnet.native[3.3.7].nativeLink'
• ./mill -i bench.runRegressions bench/resources/go_suite/base64Decode.jsonnet bench/resources/go_suite/base64.jsonnet bench/resources/go_suite/base64_byte_array.jsonnet
• hyperfine --warmup 10 --min-runs 50
• Confirmed JDK 17 still exposes the deprecated String#getBytes(int, int, byte[], int) method used here.

References

• Head: ba10f26a71bacd88ce0a476f9157a3f56c0f3c1d
• Latest source-built jrsonnet: 5b43fa88b8c43856dd5a2daa9c5c251153c5e14d