fix: Address code review findings for TSAN/ASAN PR

- Fix BasicApp::startIOThreads() member corruption: replace
  while(numberOfThreads_--) with a for loop to preserve the member value.
- Fix Coro.ipp non-sanitizer stack size: increase default from 1MB to 2MB
  to match yield_to.h and prevent stack overflows in deep call chains.
- Remove stale "TSAN deactivated" comment and dead activate_tsan variable
  from CI matrix generator.
- Clarify Application.cpp setup() comment to distinguish io_context threads
  from subsystem-specific threads.
- Use explicit load(std::memory_order_relaxed) at all SHAMap::ledgerSeq_
  read sites for consistency with the atomic store.
- Extract sanitizer detection into XRPL_SANITIZER_ACTIVE macro in
  sanitizers.h, replacing duplicated preprocessor blocks in Coro.ipp
  and yield_to.h.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.github/scripts/strategy-matrix/generate.py

@@ -255,21 +255,19 @@ def generate_strategy_matrix(all: bool, config: Config) -> list:
                     "sanitizers": "address,undefinedbehavior",
                 }
             )
-            # TSAN is deactivated due to seg faults with latest compilers.
-            activate_tsan = True
-            if activate_tsan:
-                configurations.append(
-                    {
-                        "config_name": config_name + "-tsan-ubsan",
-                        "cmake_args": cmake_args,
-                        "cmake_target": cmake_target,
-                        "build_only": build_only,
-                        "build_type": build_type,
-                        "os": os,
-                        "architecture": architecture,
-                        "sanitizers": "thread,undefinedbehavior",
-                    }
-                )
+            # Add TSAN + UBSAN configuration.
+            configurations.append(
+                {
+                    "config_name": config_name + "-tsan-ubsan",
+                    "cmake_args": cmake_args,
+                    "cmake_target": cmake_target,
+                    "build_only": build_only,
+                    "build_type": build_type,
+                    "os": os,
+                    "architecture": architecture,
+                    "sanitizers": "thread,undefinedbehavior",
+                }
+            )
         else:
             configurations.append(
                 {