UDS: collected minor findings (silent masking, suppress-response wait, NRC 0x21, dispose)

[dborgards]

Minor findings from section 3 of the repository review, UDS part. Source: docs/reviews/2026-09-10-repository-review.md (branch claude/repository-review-hpttwc). Line numbers refer to main @ 8ed80f6.

• DiagnosticSessionControlAsync(byte) masks the session type with 0x7F silently. A caller passing 0x83 gets 0x03 without being told.
• SendRawAsync with the suppress-positive-response bit set waits for a response that will never come, up to the full P2 timeout.
• NRC 0x21 (busyRepeatRequest) has no retry semantics — it is surfaced as a plain failure, though the NRC exists precisely to ask for a retry.
• UdsTimeoutException.Elapsed sometimes reports the budget and sometimes the measured time, depending on which path timed out.
• No functional-addressing path in the client. A functional 3E 80 has to be assembled by hand.
• UdsClientImpl.Dispose disposes the semaphore after a 5 s lock timeout while a holder still owns it, turning a slow operation into an ObjectDisposedException on another thread.