feat(runtime): revive ConcurrencyBurst per-instance under ADR 0015 invariants (from PR #53)

[ThomasK33]

Revive ConcurrencyBurst from closed PR #53's branch as a standalone, per-instance implementation — sequenced after the admission bound (#44) lands, under ADR 0015's invariants (bounded retention, honest rejection, no silent loss, delivery-preserving shaping).

Contract

• Per-instance burst batching only (cross-instance coalescing rejected — ADR 0015).
• Burst admission semantics/lifecycle decided here by code + hardening tests, not prose.
• The ~15 burst-lifecycle findings from PR docs(adr): ADR 0015 — deferred-dispatch admission bound; cross-instance coalescing rejected for now #54's review history are the ready-made hardening-test checklist: window anchor/ownership, FIFO ordering of rolled batches, lease-loss and handler-error member dispositions, shutdown drain, idle-coordinator GC, member-reference clearing, cap-reaching-member ownership.
• Preemption (OnLockConflict force path, LockForcer) is NOT in scope — rejected pending ADR 0015's formal-design bar.

Completes the accepted remainder of #7 (debounce/concurrent/lockScope shipped in #38; preemption rejected).

Blocked by: #44.

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