hsms: HSMS-GS multi-session support (E37 §11)

Connection now supports both HSMS-SS (single session — the
constructor's behaviour, unchanged) and HSMS-GS (multi-session).
add_session(device_id) registers additional sessions; each one has
its own NotSelected/Selected state and its own message/selected
handlers.  In GS mode the Select.req carries session_id=device_id;
in SS mode it stays at 0xFFFF (legacy).  Linktest/Separate remain
connection-scope per spec.

Public API additions:
  add_session(device_id)
  set_session_message_handler(device_id, h)
  set_session_selected_handler(device_id, h)
  session_state(device_id) -> State
  is_session_selected(device_id) -> bool
  send_request(device_id, msg, cb)
  send_data(device_id, msg)

Internal refactor: state_/on_message_/on_selected_ folded into a
SessionSlot map keyed by device_id; SS-style getters/setters route
through the primary session.  T7 + linktest are connection-scope —
T7 fires only when no session is selected; linktest runs while at
least one is.

Five wire-level tests:
  - passive: two sessions selected independently via Select.req
    with their own session_id
  - GS Select.req for an unregistered session id is Rejected
    (EntityNotSelected)
  - data routed by session_id; data on a not-selected session is
    Rejected
  - active: two registered sessions both end up selected via
    serialized Select.req per session
  - SS legacy: existing single-session API still works (session_id
    0xFFFF in Select.req)

COMPLIANCE.md §1 updated: HSMS-GS row goes ⬜ → ✅.

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

COMPLIANCE.md

@@ -41,8 +41,8 @@ Legend:
 | T6 control transaction timeout        | ✅     | E37 §10  | One concurrent control transaction. |
 | T7 not-selected timeout (passive)     | ✅     | E37 §10  | Armed on connect / on Deselect.req. |
 | T8 intercharacter timeout             | ✅     | E37 §10  | Bounds the payload read after length prefix. |
-| HSMS-SS (single-session)              | ✅     | E37 §11  | The codebase is HSMS-SS only by design. |
-| HSMS-GS (general-session)             | ⬜     | E37 §11  | Multi-session; out of scope for this revision. |
+| HSMS-SS (single-session)              | ✅     | E37 §11  | Default mode: the constructor registers a single session. |
+| HSMS-GS (general-session)             | ✅     | E37 §11  | `Connection::add_session(device_id)` registers extra sessions; per-session SELECTED state + message handlers; Select.req carries session_id=device_id in GS mode. |
 
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