perf: chunk long string byte escaping

[He-Pin]

Motivation:

Split the JMH-positive, JDK17/JIT/GC-friendly long-string rendering piece out of #776. Keep this PR focused on byte rendering for long strings that contain JSON escapes; this does not include the broader format, stdlib, compareStrings, or Scala Native experiments from #776.

Modification:

• Add CharSWAR.findFirstEscapeChar(byte[], from, to) on JVM, Scala.js, and Scala Native.
• In BaseByteRenderer, keep the existing UTF-8 byte array for long strings, locate escape bytes, bulk-copy clean chunks with System.arraycopy, and escape only matching bytes inline.
• Precompute the exact escaped output length, reserve ByteBuilder once, then write directly to the backing byte array. This removes repeated ensureLength/appendUnsafeC calls from the dirty long-string loop.
• Use a static byte hex table for \u00XX control escapes.

JIT / GC shape:

• Hot code stays in simple while loops, System.arraycopy, and small private helpers.
• No reflection, no internal JDK APIs, no closures/iterators in the rendering loop.
• No per-chunk or per-escape objects are allocated by this follow-up; the existing per-long-string UTF-8 byte array remains the only temporary for this path.
• I tested a no-allocation ASCII scalar path, but rejected it because it regressed large_string_template and large_string_join JMH.

Notable results only:

JMH target run, same machine, same command shape on upstream/master and this branch:

./mill -i bench.runRegressions bench/resources/cpp_suite/large_string_template.jsonnet bench/resources/cpp_suite/large_string_join.jsonnet

Benchmark	upstream/master	PR	Delta
large_string_template	1.552 ms/op	1.154 ms/op	-25.6% / 1.34x faster

Scala Native hyperfine, release-full native binary, 20 runs:

Benchmark	upstream/master	PR	Delta
large_string_template	10.5 +/- 0.2 ms	9.6 +/- 0.3 ms	-8.6% / 1.09x faster

large_string_join was rechecked as a guardrail and stayed neutral, so it is intentionally omitted from the result tables.

Verification:

• ./mill -i 'sjsonnet.jvm[3.3.7].compile'
• ./mill -i 'sjsonnet.jvm[3.3.7].test'
• ./mill -i 'sjsonnet.js[3.3.7].compile' 'sjsonnet.native[3.3.7].compile'
• ./mill -i 'sjsonnet.native[3.3.7].nativeLink'
• ./mill -i __.checkFormat
• git diff --check
• Focused JMH and Native hyperfine commands above

References:

• Split from perf: comprehensive Scala Native render pipeline optimization #776
• Base: b4c667d55d82d7c50c2103db967c33bebb0c2c98
• Head: ff70b63e