replace deprecated gen_fsm with gen_statem

[tsw2k]

gen_fsm has been deprecated since OTP 20 and every build emits deprecation
warnings for gtt_as_fsm and its callers. This converts it to gen_statem in
state_functions callback mode, following the shape already used by the tcap
ITU modules.

What changed

• gtt_as_fsm — down/2, inactive/2, active/2 and pending/2 become
  /3 taking a leading event type. Every event this process receives arrives as
  cast, so the clause bodies are untouched; only the heads changed.
  callback_mode/0 returns [state_functions], and a state() type was added
  for the callback specs.
• gtt_m3ua_cb — the five catch gen_fsm:send_event(AS, ...) notifications
  become catch gen_statem:cast(AS, ...).
• gtt_as_fsm_sup — the child spec starts gen_statem:start_link/3.

Notes for review

• The no-op all-state callbacks were dropped, not carried over. gtt_as_fsm
  had handle_event/3 and handle_sync_event/4 that both returned
  {next_state, StateName, StateData} unchanged. Nothing sends all-state or
  synchronous events to this process, so rather than turn them into a
  handle_event/4 that state_functions mode would never invoke, they were
  removed. If you would rather keep the symmetry with the ITU modules, say so
  and I will add it back.

• handle_info/3 became a per-state info clause. Each state function now
  ends with StateName(info, _EventContent, _StateData) -> keep_state_and_data.,
  matching both the old no-op and the pattern in tcap_tsm_fsm and
  tcap_dha_fsm.

• Unexpected events still crash. Because the info clause is the only
  catch-all, an unhandled cast in a given state still fails with
  function_clause, exactly as before. That behaviour is reachable today — for
  example 'M-ASP_ACTIVE' in the down state, or 'M-NOTIFY' at an sg-role
  AS — so it was preserved deliberately rather than softened into a silent
  no-op.

• pending and timer T(r) are unchanged. pending(timeout, ...) is carried
  over as-is, and the ?Tr macro is still unused: no transition into pending
  arms a timer, so that clause remains unreachable. Wiring up T(r) looked like a
  separate decision, so it is left alone here.

Testing

• Compiles clean on OTP 29 against m3ua and sccp. Comparing the warning sets
  before and after: the eleven gen_fsm deprecation warnings are gone, no new
  warning appears, and the nine unrelated pre-existing warnings are unchanged.
• dialyzer reports exactly the same findings as before the change (the
  pre-existing m3ua:register/7, m3ua:cast/9 and status/4 contract
  warnings), with only line numbers shifted.
• xref reports no undefined function calls.
• Not run: the SCTP-dependent paths were not exercised. macOS provides no
  SCTP (gen_sctp:open/1 returns {error, eprotonosupport}), so anything that
  needs a live M3UA association should be checked on Linux before merging.