E1: SubstrateStateMachine + SubstrateStore (E90 §6)

Per-substrate dual FSM with two orthogonal axes:

  Location (STS):
    AtSource -> AtWork (Acquire) -> AtDestination (Release)
    AtWork  -> AtSource (Return; processing aborted before completion)

  Processing:
    NeedsProcessing -> InProcess (Start) -> Processed (End)
    InProcess -> {Aborted, Stopped, Rejected, Lost} terminal
    NeedsProcessing -> {Skipped, Lost} terminal

Wire-byte values pinned via static_assert to E90-0716 §10.3.

SubstrateStore mirrors the CarrierStore pattern: non-movable, per-row
SubstrateStateMachine heap-allocated with handlers dispatching through
the store's location/processing callbacks; fire_location_event accepts
an optional new_location string so the application can carry
equipment-specific module names alongside the FSM state.

Joins EquipmentDataModel alongside carriers / load_ports.  9 test
cases cover initial state, full location lifecycle, all five
processing exits, and store-level dual-axis observer firing.

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

CMakeLists.txt

@@ -56,6 +56,7 @@ add_library(secsgem
   src/gem/exception_state.cpp
   src/gem/carrier_state.cpp
   src/gem/load_port_state.cpp
+  src/gem/substrate_state.cpp
   src/gem/host_handler.cpp
   src/config/loader.cpp
   src/endpoint.cpp
@@ -102,6 +103,7 @@ add_executable(secsgem_tests
   tests/test_exceptions.cpp
   tests/test_carrier_state.cpp
   tests/test_carriers.cpp
+  tests/test_substrates.cpp
 )
 target_link_libraries(secsgem_tests PRIVATE secsgem doctest::doctest)
 target_compile_definitions(secsgem_tests PRIVATE