feat(rawcan): callback-style Subscribe(onNext, predicate)

[dborgards]

Summary

• Adds CanBusServiceExtensions.Subscribe(onNext, predicate, bufferCapacity): a small, additive-only extension method for subscribers who want a callback instead of driving ISubscription.Frames themselves.
• Deliberately scoped to raw-CAN only, not a cross-protocol unification -- see commit message for the reasoning (protocol layers already have their own typed event APIs; unifying them would either duplicate for one line of savings or flatten typed payloads to a lowest common denominator).
• Built entirely on the existing pull API, so it inherits FR-RAW-011 for free: a slow onNext only falls behind and drops its own subscription's oldest frames, never delaying other subscriptions or the bus's own FrameObserved event.
• A throwing onNext is isolated per frame and routed through the existing BackgroundExceptionOccurred fault channel (small internal CanBusService.RaiseBackgroundException helper added, also de-duplicates the identical inline try/catch OnFrameObserved already had).

Test plan

• 4 new tests: filter correctness, slow-handler-does-not-block (mirrors the existing raw-subscription FR-RAW-011 test), Dispose stops delivery, handler-exception routed + delivery continues
• Public API surface changed (new type in CanKit.Pro.RawCan) -- approval baseline updated in the same commit
• Full dotnet build + dotnet test -- 414/414 passing

[cursor]

PR Summary

Low Risk
Additive public API and internal fault helper on the raw-CAN subscription layer; behavior is covered by extensive tests and does not change the existing pull subscription contract.

Overview
Adds a callback-style ICanBusService.Subscribe(onNext, predicate, bufferCapacity) extension in CanKit.Pro.RawCan for callers who want filter + handler instead of pumping ISubscription.Frames. Delivery runs on a dedicated background task over the existing pull subscription, so FR-RAW-011 behavior is unchanged: a slow handler only drops its own buffered frames and does not block other subscribers or FrameObserved.

Dispose completes the underlying subscription, uses an AsyncLocal reentrancy guard so self-dispose from onNext does not deadlock on pump join, and checks a disposed flag so queued channel items are not delivered after dispose returns. Handler exceptions are isolated per frame; when the service is CanBusService, they go through BackgroundExceptionOccurred via a new internal RaiseBackgroundException helper that also replaces the duplicated inline invoke in frame dispatch.

Public API approval baseline is updated; tests cover filtering, non-blocking slow handlers, dispose semantics (including in-handler dispose), exception surfacing, and continued delivery.

^{Reviewed by Cursor Bugbot for commit fa54874. Bugbot is set up for automated code reviews on this repo. Configure here.}

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

Autofix Details

Bugbot Autofix prepared a fix for the issue found in the latest run.

• ✅ Fixed: Dispose from callback hangs two seconds
  • CallbackSubscription.Dispose now skips the pump-task join when invoked from onNext, so unsubscribe-from-handler no longer deadlocks until the two-second timeout.

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[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

Bugbot Autofix prepared a fix for the issue found in the latest run.

• ✅ Fixed: Self-dispose still delivers buffered frames
  • The callback pump now checks the disposed flag around onNext so a self-dispose no longer drains remaining buffered frames after Dispose returns.

_{You can send follow-ups to the cloud agent here.}

^{Reviewed by Cursor Bugbot for commit e44f3cc. Configure here.}