feat(agera): derive mounted state from the graph instead of counting subscribers

[dangreen]

Holistic rework of the signal lifecycle: the mounted state is no longer a counter maintained on every node, it is derived from the graph on demand.

• replace the subscriber counters with a presence model: mounted state is never stored, it is a question asked of the graph - does this node have a live path up to an effect - memoized per drain and invalidated by any edge edit
• deliver mount and unmount as one level change from a coalescing queue at quiescent boundaries: churn inside a turn is silent, sources mount before their dependents and unmount after them
• move the layer behind two lazy sockets wired on first mountable() use, so a signal-only bundle drops it entirely: signal -17%, nanoviews -4%
• onMounted and the new isMounted live in signal.ts over a plain listener list; node.subsCount and node.mounted are gone
• an onMounted listener registered while the signal is already mounted now gets true at the next boundary instead of waiting for the next cycle
• fix late contagion and late mountable() never mounting a signal, a cold computed unmounting a signal under a live subscriber, stuck mounted(true) on discarded subscriptions, listeners skipped or served twice when one un-subscribes or registers during a fire, and delivery lost for other signals when a listener throws

Sizes (brotli, vs the previous mechanism): agera { signal } 1606 -> 1332, minimal set 1791 -> 1503, popular set 1917 -> 1917, all publics 2630 -> 2715; nanoviews all publics 7126 -> 6807, average usage 4330 -> 4049.

Verified with the full test chain (agera 125 + 1 expected fail, kida 61, store 57, nanoviews 72, query 162, router 129), size-limit across all packages, randomized differential fuzzing of presence, ordering and listener interleavings, and a real-browser benchmark at parity.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.54%. Comparing base (dd3169f) to head (2cc1f8c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #191      +/-   ##
==========================================
+ Coverage   84.23%   84.54%   +0.30%     
==========================================
  Files         138      139       +1     
  Lines        3026     3112      +86     
  Branches      565      580      +15     
==========================================
+ Hits         2549     2631      +82     
- Misses        339      343       +4     
  Partials      138      138              

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