Feature/core improvements

[hexplus]

Description

Hardens the reactivity core to close four real correctness gaps surfaced by a deep audit:

• Effect re-entry: effects that write to a signal they subscribe to no longer silently drop the update — they re-run after the current body via a rerunPending flag, with a 100-iteration safety cap to bail loudly on true cycles.
• Stale-dep leak in derived(): conditional getters like () => flag() ? a() : b() no longer keep both branches' signals subscribed forever. retrack() now tags each dep edge with a per-evaluation epoch and unsubscribes stale edges at end of run.
• Sibling-effect convergence: the fast and slow paths of notifySubscribers used to diverge on whether cross-writing effects could re-run; unified into a single drain with dedup-cleared-before-invoke so sibling effects reliably converge on consistent state.
• Cycle detection: replaced the coarse 100k-total-iteration cap (which conflated real cycles with large legitimate fan-out) with per-subscriber repeat counting. Default cap 50, configurable via new setMaxSubscriberRepeats(n).

Also: empty __s subscriber sets are now released, subscriberStack shrinks when idle, and __f/__s fast-path bookkeeping is centralized in a syncFastPath() helper.

Internal _deps storage migrated from Set<signal> to Map<signal, epoch> to carry the epoch tags. Public API is unchanged; devtools introspect.getDependencies() updated for the new layout.

201/201 test files, 2187/2187 tests passing. No behavior changes to user code that was already correct.

Related Issue

Closes #

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Checklist

• I have read CONTRIBUTING.md
• My code builds without errors
• I have tested my changes
• I have updated documentation if needed