perf: validate actor path elements without String.charAt

[pjfanning]

Motivation

ActorPath.findInvalidPathElementCharPosition runs for every named actorOf. It scanned the name with String.charAt inside a pattern match with guards, plus a ValidSymbols.indexOf for every non-alphanumeric character.

String.charAt inlines to isLatin1() ? StringLatin1.charAt : StringUTF16.charAt, and that branch is profiled once, JVM-wide, in String.charAt's own bytecode. Any non-ASCII string handled anywhere in the process (a Jackson payload, a log line) pollutes that profile, after which C2 compiles both coders into every charAt loop. Measured locally, the validator halves in throughput once the profile is polluted.

Modification

• Copy the element out once with getBytes(ISO_8859_1) — an intrinsic array copy for Latin-1 strings — and scan the byte[] with a 128-entry flag table (valid-char bit, hex-digit bit) instead of calling charAt per character. Characters outside Latin-1 encode as ? and 0x80–0xFF as negative bytes; both are invalid, so the accepted set and the reported position are unchanged.
• Add directional ActorPathSpec tests for accepted names, rejected names and the reported error position, including Latin-1, non-Latin-1 and surrogate-pair input.
• Extend ActorPathValidationBenchmark with a polluted param that pre-warms String.charAt with UTF-16 strings, and keep the previous charAt-based validator there as charAtLoop* for comparison.

Result

Name validation no longer depends on the String.charAt coder profile. Local JMH (-f 1, short run, noisy machine — the trend is what matters):

Benchmark	polluted=false	polluted=true
charAtLoopActor_1 (previous code)	101 ops/µs	49 ops/µs
handLoopActor_1 (this PR)	139 ops/µs	162 ops/µs
charAtLoop7000 (previous code)	0.197 ops/µs	0.107 ops/µs
handLoop7000 (this PR)	0.176 ops/µs	0.142 ops/µs

I tried the same treatment on the other hot charAt users, MurmurHash.stringHash and Helpers.base64, and measured no benefit (hash arithmetic and StringBuilder.append dominate; toCharArray only adds an allocation), so those are left as they are.

Tests

• sbt "actor-tests/testOnly org.apache.pekko.actor.ActorPathSpec org.apache.pekko.actor.LocalActorRefProviderSpec" — 25 passed
• sbt "actor-tests/testOnly org.apache.pekko.routing.ConsistentHashingRouterSpec" — passed
• sbt "actor/mimaReportBinaryIssues" — no issues
• sbt "bench-jmh/Jmh/compile" and Jmh/run of ActorPathValidationBenchmark
• scalafmt on changed Scala files, git diff --check

References

None - performance follow-up to the hand-written actor name validator