perf: speed up constant string joins

[He-Pin]

Summary

• Keep large constant std.makeArray(... literal) arrays as an O(1) constant array view.
• Fast-path std.join over constant/repeated string arrays with exact StringBuilder sizing.
• Mark ASCII JSON-safe joined/repeated strings so byte rendering can skip escape scanning and UTF-8 encoding.
• Add JVM/Native UTF-16-lane SWAR ascii-safe checks; Scala.js keeps a scalar fallback.
• Drop remaining private[this] in touched Scala sources.

Benchmarks

Baseline: upstream/master (04e5707c). This PR: 7dd77cb7.

Full JMH sweep was run on both sides with ./mill -i bench.runRegressions. Only the standout result is listed here.

Benchmark	upstream/master	This PR	Delta
bench/resources/cpp_suite/large_string_join.jsonnet	0.542 ms/op	0.268 ms/op	-50.6%

GC profiler for the same benchmark:

Metric	upstream/master	This PR	Delta
gc.alloc.rate.norm	1,531,405 B/op	633,079 B/op	-58.7%

Scala Native hyperfine (--shell=none --warmup 5 --runs 50, output via -o /dev/null):

Command	Mean
sjsonnet native, upstream/master	5.0 ms
sjsonnet native, this PR	4.0 ms
jrsonnet	3.4 ms

This narrows the large_string_join Scala Native gap to jrsonnet from ~1.45x slower to ~1.15x slower.

Validation

• ./mill -i __.checkFormat
• git diff --check
• ./mill -i 'sjsonnet.jvm[3.3.7].test'
• ./mill -i 'sjsonnet.js[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'