secsi: T3 / T4 enforcement moved into the FSM

The SECS-I Protocol FSM now enforces T3 (reply timeout) and T4
(inter-block timeout) directly, instead of leaving them as
upper-layer hooks.

T3: on complete_send, if the block we just acked had W=1, record its
system_bytes in awaiting_reply_sys_ and emit ActionStartTimer{T3}.
deliver_recv cancels T3 when a block arrives whose system_bytes
match the outstanding request.  EventTimeout{T3} aborts the FSM with
"T3 reply timeout".

T4: deliver_recv emits ActionStartTimer{T4} whenever the delivered
block has end_block=false.  The next block's deliver_recv cancels
the timer; EventTimeout{T4} aborts with "T4 inter-block timeout".

abort() now also cancels T3/T4 and clears the tracking state.

Test changes:
  - Old "T3/T4 are FSM-level no-ops" test → REPLACED by four new
    tests: T3 arm+expire, T3 arm+matching-reply cancels, T4
    arm+expire, T4 arm+next-block cancels.
  - Two new observer accessors on Protocol (awaiting_reply,
    awaiting_next_block) so the tests can assert tracking state
    without poking internals.

COMPLIANCE.md §1a: T3 + T4 rows go ⬜ → ✅.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

COMPLIANCE.md

@@ -57,8 +57,8 @@ Legend:
 | RTY retry counter                     | ✅     | E4 §10.2 | Per-block retry budget, exhaust → ActionRaiseError. |
 | T1 inter-character timer hook         | ✅     | E4 §10.1 | Drained in `RecvBlock`; host wires the actual asio timer. |
 | T2 protocol timer hook                | ✅     | E4 §10.1 | Triggers a retry from any send state. |
-| T3 reply timer                        | ⬜     | E4 §10.1 | Driven by the upper layer (same as HSMS). |
-| T4 inter-block timer                  | ⬜     | E4 §10.1 | Multi-block message-gap; FSM emits hook events. |
+| T3 reply timer                        | ✅     | E4 §10.1 | FSM tracks system_bytes of outstanding W=1 primaries; arms T3 on send-complete, cancels on matching reply, aborts on expiry. |
+| T4 inter-block timer                  | ✅     | E4 §10.1 | FSM arms T4 when a block delivers with end_block=false; cancels when the next block arrives, aborts on expiry. |
 | Master/slave contention resolution    | ✅     | E4 §7.1.4 | Slave yields on simultaneous ENQ; master holds. |
 | Serial port wiring (asio)             | ⬜     | —        | FSM is IO-free; serial integration is a wiring follow-up. |
 | TCP tunnel for testing                | ✅     | —        | `secsi::TcpTransport` wraps the FSM behind an asio TCP socket; mirrors secsgem-py's `secsitcp/`. |