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>

include/secsgem/gem/substrate_state.hpp

@@ -127,6 +127,15 @@ class SubstrateStateMachine {
   bool on_processing_event(SubstrateProcessingEvent e);
   bool on_id_event(SubstrateIDEvent e);
 
+  // Direct state-restore (persistence replay only).  Bypasses the
+  // transition tables and does NOT fire change handlers.
+  void restore_state(SubstrateState loc, SubstrateProcessingState proc,
+                     SubstrateIDStatus id) {
+    loc_  = loc;
+    proc_ = proc;
+    id_   = id;
+  }
+
  private:
   SubstrateTable           loc_table_;
   SubstrateProcessingTable proc_table_;