feat: scheduled wakeup primitive for delegates (ScheduleWakeup / WakeupFired)

[sanity]

Problem

Delegates have no way to schedule future execution. The OutboundDelegateMsg
enum has no ScheduleWakeup-equivalent variant and InboundDelegateMsg has no
WakeupFired, so delegate execution is purely message-driven. dApps that need
periodic background work (key rotation, TTL pruning, scheduled publication) are
forced to push it into a UI/client sync loop, which stops working when the UI is
closed. Driving use case: River private-rooms weekly secret rotation
(freenet/river#228).

Approach

Add the two wire variants the host↔delegate protocol needs:

• OutboundDelegateMsg::ScheduleWakeup { at: SystemTime, tag: Vec<u8> }
• InboundDelegateMsg::WakeupFired { tag: Vec<u8> }

Both are appended at the end of their enums (bincode variant tag 8), so the
change is wire-compatible for every existing variant — delegate WASM compiled
against an older stdlib keeps deserializing everything it already understood.

• OutboundDelegateMsg is deliberately left not #[non_exhaustive]: the
  host must consciously handle every outbound variant, so the compiler should
  force an arm for each new one. The inaccurate doc comment on
  InboundDelegateMsg that claimed OutboundDelegateMsg was already
  #[non_exhaustive] is corrected.
• InboundDelegateMsg stays #[non_exhaustive]; unknown inbound variants are
  forwarded to the delegate WASM unchanged, so WakeupFired flows through.
• These variants travel only on the bincode host↔delegate FFI path, not the
  FlatBuffers WS path, so no .fbs schema / generated-code / TypeScript changes
  are needed. ScheduleWakeup gets a "reached client serialization - this is a
  bug" arm in the WS encoder mirroring SendDelegateMessage.

Bumps 0.8.2 → 0.8.3.

Testing

New wire-format pin tests freeze the variant tags so a future reorder is caught
loudly:

• inbound_wakeup_fired_wire_format_is_stable — full byte layout at tag 8.
• outbound_schedule_wakeup_wire_format_is_stable — tag 8 + round-trip.

Downstream

Host-side implementation is in freenet-core#3972 (opens after this publishes,
per the stdlib-first release policy).

Refs: freenet/freenet-core#3972

[AI-assisted - Claude]

[sanity]

Host-side consumer: freenet/freenet-core#4666 (issue #3972). This stdlib PR must merge + publish 0.8.3 first (stdlib-first), then the core PR can build.

[AI-assisted - Claude]