Performance improvements part2

[elcritch]

Summary

This PR improves subscription lookup performance for agents with larger subscription lists while preserving the fast path for small lists.

• Keeps subcriptions sorted by signal name on insert.
• Adds binary-search range lookup for larger subscription lists.
• Uses a linear-scan fast path below sigilsSubscriptionBinarySearchThreshold to avoid regressing the common single-signal case.
• Applies sorted insertion to AgentActor subscriptions as well.
• Adds a regression test for sorted signal ranges plus AnySigilName wildcard lookup.
• Adds tests/tbenchmarks3.nim, a multi-signal fanout benchmark with 6 signals x 6 slots.

Performance Notes

The small-list threshold keeps tests/tbenchmarks.nim close to the previous single-subscription behavior:

emit->slot procRatio ~= 18-19x
methodRatio ~= 13x

The new multi-signal benchmark stresses 36 subscriptions and exercises the binary-search path:

signals=6, slotsPerSignal=6, subscriptions=36
emits=600000, slotCalls=3600000

Local samples showed the multi-signal emit loop dropping from the earlier linear-scan sample around 283 ms to roughly 58-73 ms.

Testing

Ran:

nim c -r tests/tslots.nim
nim c -r tests/tclosures.nim
nim c -r tests/tweakrefs.nim
nim c -r tests/tregistryCloneSignals.nim
nim c -r tests/tthreadPool.nim
nim c -r -d:release tests/tbenchmarks.nim
nim c -r -d:release tests/tbenchmarks2.nim
nim c -r -d:release tests/tbenchmarks3.nim
git diff --check