feat(scheduler-allocation): clear unsatisfiable-resreq cache per session

Clear the cache inside core_begin_session_internal() so a job cached as
unsatisfiable in one session is re-evaluated fresh in the next (Req 7.2).

An initial weak-symbol cast-based approach was rejected: it corrupted
xce_sched_sim_driver_t.alt_events_file, since core_begin_session_internal()
is shared between driver.c (real xce_scheduler_t) and sim_loop.c (a
structurally different, deliberately isolated sim driver struct with no
cache field at the same byte offset) -- the strong cache_clear() symbol
won unconditionally for both, invisible to ASAN and untested by any
test_sim_*.c file.

Fixed via a driver-registered callback: xce_sched_core_t gains a
cache_clear_cb hook (NULL by default, per xce_sched_core_init()'s
full-struct memset), set only in main.c's driver-mode branch --
mirroring the existing on_decision/xce_core_set_decision_cb() pattern.
--sim mode returns before that registration is ever reached, so its hook
stays NULL and core_begin_session_internal()'s guarded call is a no-op.
The real xce_scheduler_t-casting adapter lives in alloc_cache.c rather
than driver.c, since ~11 tests white-box-include driver.c without
linking alloc_cache.c.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XYdtrEWWZfKRRGED7raEYg

src/scheduler/include/sched/alloc_cache.h

@@ -131,4 +131,14 @@ extern int cache_insert(xce_scheduler_t* sched, xce_sched_obj_t* obj,
  */
 extern void cache_clear(xce_scheduler_t* sched);
 
+/*
+ * xce_alloc_cache_clear_cb - the xce_core_cache_clear_cb_t (core.h)
+ * adapter for cache_clear() above (scheduler-allocation spec, task 5.2
+ * round 2, Req 7.2). Registered on the real xce_scheduler_t's core via
+ * xce_core_set_cache_clear_cb() (main.c). See alloc_cache.c's own doc
+ * comment on this function for the full rationale, including why it lives
+ * here rather than in driver.c.
+ */
+extern void xce_alloc_cache_clear_cb(xce_sched_core_t* core);
+
 #endif