[Bug] BoundedMailbox drop-head overflow is a no-op while the mailbox is suspended

[pathosDev]

Rationale

BoundedMailbox implements drop-head overflow via super.dequeueUser() (src/mailbox/BoundedMailbox.ts:43-48). But Mailbox.dequeueUser() returns undefined without removing anything while the mailbox is suspended (src/internal/Mailbox.ts:49-52) — e.g. during a supervision restart or while a stash gate holds the actor. The overflow branch then still enqueues the new envelope:

• the "bounded" queue grows past capacity for as long as the mailbox stays suspended under a flooding producer, and
• droppedCount / onDrop (and any metric built on them) report drops that never happened.

Scope

• Drop from the underlying buffer regardless of suspension (e.g. a protected removeOldest() on Mailbox that bypasses the suspended gate) — drop-new and reject semantics stay unchanged.
• Test: enqueue past capacity while suspended → size === capacity and droppedCount matches actual removals; resume → normal delivery order.

Note (current state, v0.11): overflow values are 'drop-head' | 'drop-new' | 'reject' (default 'reject'), and BoundedMailboxOptions + BoundedMailboxOptionsValidator already exist — this issue only touches the suspended-drop logic.

Documentation

• CHANGELOG Fixed entry.
• Mailbox docs note (EN + DE) on overflow semantics during suspension.

Acceptance

• Capacity invariant holds under suspend/resume; drop metric matches reality; existing mailbox tests green.

Relates

#310 (bounded default mailbox). Improvement program M1/S4.