persistence: SubstrateStore enable_persistence(dir)

Same pattern as carriers: per-substrate binary record (.sub) with
atomic .tmp+rename, replay on enable, delete on remove. Records
current state across all three E90 axes (location / processing /
ID-status), plus substid / carrierid / slot / free-form location
label. History is deliberately NOT journaled — it's an in-memory
ring buffer and rebuilding from replayed state would mislead.

Five new tests cover full-axis replay, every terminal processing
state, remove-deletes-journal, corrupt-record drop, and the
history-is-transient invariant.

Closes #2 in the test-gap backlog.

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

CMakeLists.txt

@@ -113,6 +113,7 @@ add_executable(secsgem_tests
   tests/test_carriers.cpp
   tests/test_carrier_persistence.cpp
   tests/test_substrates.cpp
+  tests/test_substrate_persistence.cpp
   tests/test_ept.cpp
   tests/test_cem_objects.cpp
   tests/test_modules.cpp