v1.1.0: Event Bus

[1.1.0] - Event bus core

Groundwork for the upcoming event-driven execution mode: the C core of the event bus. It is not
exposed to PHP yet — an inert foundation with its own CI-gated test suite — so nothing you can call
from a script changes in this release.

Added

• The event bus C core. An event is a slot from a preallocated pool: fixed TINY / SMALL /
  MEDIUM size classes, free-list based, with no allocation on the hot path. When a class is
  exhausted, acquiring a slot fails instead of blocking or allocating — the pool is the backpressure —
  and a per-tag in-use counter acts as a leak oracle. On top of the pool: dispatch() (queue an
  event; ownership of the slot transfers to the queue) and now() (call the listeners inline, no
  transfer); a dispatch-frame stack that returns every slot a frame still holds when it exits —
  normally or through a zend_bailout unwind, across nested frames — so a handler that throws never
  leaks its slot; two cascade guards (a queue-hop depth that drops the over-limit event, and a
  nested-now() depth that refuses the call before it can overflow the task stack); and a small,
  separate ISR reserve with a drop-and-count policy for events emitted from interrupt context. The
  core is portable C — it compiles and runs on the host, no hardware needed.
• Event bus test gate in CI. The pool-exhaustion and automatic-release suite — exhaustion per
  size class, an exception mid-dispatch, a cascade that unwinds across frames, an intermediate
  listener that throws (the slot is released exactly once), no leaks over thousands of mixed cycles,
  plus the cascade counters and the ISR reserve — builds and runs on the host with gcc -Werror on
  every push and pull request.