perf: use ASCII bitsets for strip chars

[He-Pin]

Motivation:

std.stripChars, std.lstripChars, and std.rstripChars now already avoid boxed Set[Int] for single/BMP delimiter sets on master. This PR keeps that Unicode-aware path, and narrows the ASCII-only multi-character delimiter case further by replacing the per-call java.util.BitSet allocation with primitive masks.

Key Design:

• ASCII-only delimiter sets use primitive masks before the BMP fallback.
• JVM and Scala.js use four Int masks; Scala Native uses two Long masks.
• Non-ASCII BMP delimiter sets keep the upstream BitSet fast path.
• Surrogate-containing delimiter sets still fall back to codepoint iteration.
• chars is forced before str, preserving existing error evaluation order.

Benchmark Results:

Rebased onto upstream/master at 8b67cb1e (Add workaround to make CI more reliable (#824)).

JMH full regression command:

./mill -i bench.runRegressions

Full JMH ran all 45 regression cases on master and this PR. There is no stable standout runtime delta after rebase. The strip-char cases are within normal local JMH noise:

• lstripChars.jsonnet: 0.111 -> 0.112 ms/op
• rstripChars.jsonnet: 0.113 -> 0.114 ms/op
• stripChars.jsonnet: 0.111 -> 0.110 ms/op

Native hyperfine command shape:

hyperfine --shell=none --warmup 5 --runs 50 --time-unit millisecond \
  -L case lstripChars.jsonnet,rstripChars.jsonnet,stripChars.jsonnet \
  '<master-native> bench/resources/go_suite/{case}' \
  '<this-pr-native> bench/resources/go_suite/{case}' \
  '<jrsonnet> bench/resources/go_suite/{case}'

Native CLI also has no standout delta vs current master:

• lstripChars.jsonnet: 3.81 -> 3.80 ms; jrsonnet 2.00 ms
• rstripChars.jsonnet: 3.74 -> 3.81 ms; jrsonnet 2.02 ms
• stripChars.jsonnet: 3.82 -> 3.87 ms; jrsonnet 2.13 ms

Analysis:

The earlier positive JMH signal disappeared after rebasing because current master already has the BMP BitSet strip fast path. This PR should be treated as a small GC-friendly allocation cleanup for ASCII delimiter sets, not as a visible improvement on the go_suite strip benchmarks or the jrsonnet gap.

Validation:

• ./mill -i __.checkFormat: pass
• git diff --check: pass
• ./mill -i 'sjsonnet.jvm[3.3.7].test': pass
• ./mill -i 'sjsonnet.js[3.3.7].test': pass
• ./mill -i 'sjsonnet.native[3.3.7].test': pass
• ./mill -i bench.runRegressions: pass on master and this PR
• Native hyperfine --shell=none --warmup 5 --runs 50: pass

[He-Pin]

Closing as low priority after rebase: current master already has the main strip-char fast path, and this PR shows no stable standout delta on the go_suite strip docs cases or Native hyperfine.

[He-Pin]

Reopened. This is not negative; keep it as draft as a small GC-friendly strip delimiter cleanup. Current docs strip benchmarks do not show standout gains, so it should not be moved to ready yet.