WASM adapter: implement buffer metrics (observability parity)

[lxsaah]

With the observability feature enabled, the Tokio and Embassy adapters
report per-buffer metrics (produced/consumed/dropped counters plus current
occupancy) through Buffer::metrics_snapshot(). The WASM adapter doesn't:
WasmBuffer never embeds the shared BufferCounters and doesn't override
metrics_snapshot(), so the trait's default None kicks in and
observability silently has a blind spot on WASM targets. The crate doesn't
even expose an observability feature yet.

This is a pattern-following task: both other adapters show exactly how the
wiring looks, and the counters type is already portable.

Where to look

• aimdb-core/src/buffer/counters.rs — BufferCounters, built on portable-atomic; compiles fine on wasm32
• aimdb-core/src/buffer/traits.rs — the Buffer::metrics_snapshot() / reset_metrics() defaults and the BufferMetrics trait to implement
• aimdb-tokio-adapter/src/buffer.rs and aimdb-embassy-adapter/src/buffer.rs — the pattern to mirror: an Arc<BufferCounters> field behind #[cfg(feature = "observability")], increments in the push/receive paths, occupancy computed per buffer variant
• aimdb-wasm-adapter/src/buffer.rs — WasmBuffer, the Rc<RefCell<…>> single-threaded design this lands in

What to do

• Add an observability feature to aimdb-wasm-adapter/Cargo.toml that
  forwards to aimdb-core/observability (see how the other adapters do it).
• Embed BufferCounters in WasmBuffer (an Rc fits the adapter's
  single-threaded Rc<RefCell<…>> design).
• Count events with the same semantics as the Tokio and Embassy adapters:
  produced on every push, consumed on every successful receive, and
  dropped only at reader-side lag detection on the ring — when a reader
  discovers it has fallen behind and skips ahead, add the skipped count.
  Tokio does this where it maps broadcast::error::RecvError::Lagged(n),
  Embassy where it maps WaitResult::Lagged(n), and the WASM ring reader
  already computes exactly this lag_count when a reader falls behind.
  SingleLatest and Mailbox overwrites are deliberately not counted as
  drops in either reference adapter — keep that parity, even though the WASM
  buffer stores values directly and could detect them.
• Occupancy per variant: ring length for SpmcRing; value present → 1 for
  SingleLatest and Mailbox (the WASM buffer stores values directly, so it
  can report honest occupancy).
• Implement BufferMetrics and override metrics_snapshot() /
  reset_metrics() behind the feature gate.
• Add tests mirroring the Tokio adapter's feature-gated metrics_tests
  module. The WASM adapter's buffer tests run on the host
  (cargo test -p aimdb-wasm-adapter --no-default-features --lib), so no
  browser or wasm-pack setup is needed for this task.
• Extend the Makefile test target with a second WASM host-lib run adding
  --features observability, mirroring how it already tests the Tokio
  adapter both with and without the feature — otherwise the new gated tests
  never run in make check.

Done when

• With observability enabled, metrics_snapshot() returns Some(...) for
  all three buffer types on the WASM adapter
• Counter semantics match the Tokio and Embassy adapters for the same
  sequence of operations (covered by tests)
• make check is green (it includes the WASM host-lib test lane)