docs: refresh stale file paths after store/ reorg + gen_messages rename

generate_messages.py → gen_messages.py and several gem/ headers moved
under gem/store/ (carrier_store.hpp → store/carriers.hpp, etc.);
e84.hpp split into e84_state.hpp.  The guided-tour chapters still
pointed at the old paths — relink them so the deep-link footnotes
resolve.

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

docs/02_the_cast.md

@@ -290,11 +290,10 @@ substrate.
 
 **Where it lives in this codebase.**
 
-- The E84 handshake state machine: [`include/secsgem/gem/e84.hpp`](../include/secsgem/gem/e84.hpp)
-  + [`src/gem/e84.cpp`](../src/gem/e84.cpp).
-- The TA1/TA2/TA3 timer wiring: [`include/secsgem/gem/e84_timers.hpp`](../include/secsgem/gem/e84_timers.hpp),
-  [`include/secsgem/gem/e84_asio_timers.hpp`](../include/secsgem/gem/e84_asio_timers.hpp).
-- The per-port store: `e84_ports.hpp` (see [`include/secsgem/gem/e84_ports.hpp`](../include/secsgem/gem/e84_ports.hpp)).
+- The E84 handshake state machine: [`include/secsgem/gem/e84_state.hpp`](../include/secsgem/gem/e84_state.hpp)
+  + [`src/gem/e84_state.cpp`](../src/gem/e84_state.cpp).
+- The TA1/TA2/TA3 timer wiring (asio): [`include/secsgem/gem/e84_asio_timers.hpp`](../include/secsgem/gem/e84_asio_timers.hpp).
+- The per-port store: [`include/secsgem/gem/store/e84_ports.hpp`](../include/secsgem/gem/store/e84_ports.hpp).
 - Tests covering the timing rules: [`tests/test_e84.cpp`](../tests/test_e84.cpp),
   [`tests/test_e84_timers.cpp`](../tests/test_e84_timers.cpp),
   [`tests/test_e84_asio_timers.cpp`](../tests/test_e84_asio_timers.cpp),