Add a threading.RLock (re-entrant lock) to InvariantService that guards
all shared mutable state (_invariants dict and _enforcement_records list).
Every public method acquires the lock before reading or writing shared data.

This prevents RuntimeError: dictionary changed size during iteration and
data corruption under concurrent plan execution when the service is registered
as a DI Singleton shared across threads.

PR Compliance Checklist:
- [x] CHANGELOG.md - added entry under [Unreleased]/Fixed section
- [x] CONTRIBUTORS.md - updated HAL 9000 contribution with thread safety mention
- [x] Commit footer included (ISSUES CLOSED: #8209)
- [x] BDD tests - new feature file and step definitions for thread safety
- [x] Epic reference - fix(concurrency) type follows project convention

@@ -0,0 +31,4 @@
+  # -----------------------------------------------------------------
+
+  Scenario: Concurrent list from multiple threads does not raise RuntimeError
+    Given 20 invariants pre-added to the service

@@ -0,0 +63,4 @@
+  # -----------------------------------------------------------------
+
+  Scenario: Concurrent enforce_invariants does not raise RuntimeError
+    Given 10 invariants pre-added to the service from different scopes

@@ -0,0 +84,4 @@
+  Scenario: Shared singleton instance survives concurrent access from 20 threads
+    When 20 threads each add 3 invariants and then list them back
+    Then no exception should have been raised during concurrent singleton access
+    And the service invariant count should be non-negative

@@ -0,0 +186,4 @@
+    "{a:d} threads concurrently add invariants and "
+    "{l:d} threads concurrently list invariants"
+)
+def step_when_concurrent_add_and_list(context: Any, a: int, l: int) -> None:

@@ -0,0 +387,4 @@
+            try:
+                context.inv_service.add_invariant(
+                    text=f"Mixed-thread-{thread_idx}-inv-{i}",
+                    scope=(thread_idx % 3) + 1

@@ -0,0 +469,4 @@
+
+
+@then(
+    "the service invariant count should be non-negative and the enforcement record count should be non-negative"

@@ -0,0 +242,4 @@
+                    errors.append(exc)
+
+    threads = [threading.Thread(target=add_worker, args=(t,)) for t in range(a)]
+    threads += [threading.Thread(target=list_worker, args=(t,)) for t in range(list_threads)]

@@ -0,0 +346,4 @@
+)
+def step_then_no_exc_effective(context: Any) -> None:
+    assert not context.errors, (
+        f"Exception(s) raised during concurrent get_effective_invariants: {context.errors}"

@@ -0,0 +418,4 @@
+            try:
+                context.inv_service.add_invariant(
+                    text=f"Mixed-thread-{thread_idx}-inv-{i}",
+                    scope=[InvariantScope.GLOBAL, InvariantScope.PROJECT, InvariantScope.PLAN][thread_idx % 3],

@@ -0,0 +505,4 @@
+
+
+@then(
+    "the service invariant count should be non-negative and the enforcement record count should be non-negative"