Perf: Update composite keys (cls) in one pass, without intermediate collections

[nguyenyou]

Recreates the original PR #215, which was automatically closed when my fork was accidentally deleted.

Today

Every cls <-- signal emission ends in updateCompositeValue, which rebuilds the attribute string like this:

getAttribute → decode (split → filter → wrap → List) → filterNot → ++ → mkString → setAttribute

That's ~5 short-lived collections per update, usually for a one- or two-word string.

This PR

Default composite keys produce the same result in one pass:

• DefaultCompositeCodec.encodeUpdated(domValue, removeItems, addItems) walks the split DOM value once, skips removed items, appends added ones, and builds the string directly. It equals encode(decode(domValue).filterNot(removeItems.contains) ++ addItems) for the default codec.
• CompositeCodec.encodeUpdated calls its overridable decode and encode methods, preserving the behavior of custom codecs.
• CompositeCodec.normalize: fast path when there's no separator ("active" → List("active"), no split); otherwise builds the List directly instead of jsSplit → ew.filter → asScalaJs.toList.
• updateCompositeValue reuses keyItemsWithReason instead of looking up _compositeValues twice.

No behaviour change: the bookkeeping, the DOM read (kept for third-party classes), and the exact attribute string written are all the same as before.

Numbers

n elements with cls <-- sharedSignal, toggling the signal. fullLinkJS, Chrome 152, M3 Max, runs interleaved A/B/A/B:

rows	before	after
1000	461 µs	387 µs	1.19×
2000	1042 µs	802 µs	1.30×
5000	3290 µs	2761 µs	1.19×

These measurements predate the custom-codec compatibility follow-up; default keys still use the same one-pass implementation. Control, the same binding with class set via a plain htmlAttr: unchanged (~200 µs @ 1000 rows). So cls is still ~2× a plain attribute – mostly the DOM read, which I kept on purpose.

Testing

• Full Scala 3 suite passes (411), including a regression test for overridden codec methods.
• Scala 2.13 compilation could not be rerun in this checkout because airstream_sjs1_2.13:18.0.0-M5-SNAPSHOT is unavailable.
• Checked normalize and the default codec's encodeUpdated against the original code on 20k random inputs (extra / leading / trailing separators, duplicates, empty / unset values, " " and "," separators): identical output. I didn't add this as a test because of the "no speculative tests" note in notes/Testing.md – happy to add it if you'd like.

🤖 Generated with Claude Code