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/30_repository_tour.md

@@ -67,7 +67,7 @@ secs-gem/
 │   └── equipment.yaml       Demo SVIDs/ECIDs/CEIDs/alarms/recipes.
 │
 ├── tools/                   Build-time scripts.
-│   └── generate_messages.py Codegen: messages.yaml → messages.hpp.
+│   └── gen_messages.py      Codegen: messages.yaml → messages.hpp.
 │
 ├── interop/                 External-validator harnesses.
 │   ├── README.md            Harness-by-harness detail.
@@ -171,7 +171,7 @@ included.
 1. **Pull in dependencies** — `find_package(Threads)`,
    `find_package(yaml-cpp)`, `FetchContent` for doctest.  Standalone
    Asio is header-only (no link step).
-2. **Run codegen** — invokes `tools/generate_messages.py` to turn
+2. **Run codegen** — invokes `tools/gen_messages.py` to turn
    `data/messages.yaml` into `build/generated/secsgem/gem/messages.hpp`.
    Listed as a custom command so it re-runs when `messages.yaml`
    changes.