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

.kiro/specs/scheduler-allocation/tasks.md

@@ -79,7 +79,7 @@
   - _Requirements: 7.3, 7.4, 7.5_
   - _Boundary: scheduler.c dispatch_
   - _Depends: 2.2, 3.1, 4.2_
-- [ ] 5.2 (P) Clear the cache at the start of every scheduling session
+- [x] 5.2 (P) Clear the cache at the start of every scheduling session
   - Clear the cache as part of the existing per-session reset step, alongside the other state that step already resets
   - Observable: a job cached as unsatisfiable in one session is re-evaluated fresh (not immediately pended from the cache) in the next session
   - _Requirements: 7.2_
@@ -122,3 +122,5 @@
 - (5.1) A nested primary (inside a COMPOUND/ALTER) has no real per-primary `xce_sched_obj_t`; `sched_primary_cache_obj_()` synthesizes one on the stack per dispatch for cache-key purposes. This is memory-safe (verified line-by-line): `alloc_cache.c`'s `build_key_()` copies the fingerprint and identity fields by value synchronously during the call, before the stack frame unwinds — no pointer into the caller's stack locals is ever retained.
 - (5.1) Req 7.5's "same reason category" is only exercisable in the degenerate all-zero case today: no code path anywhere in the scheduler sets `xce_job_preason_t.main_reason` to a nonzero value except this task's own new `sched_pend_from_cached_reason_()` — every existing allocator-exhaustion path only sets host/resource sub-reasons via `add_job_preason()`. If a future task wants to prove genuine (non-degenerate) category propagation, it needs a path that actually produces a nonzero `main_reason` first.
 - (5.1) Two significant, pre-existing, out-of-scope bugs were discovered and confirmed during this task's review, logged as bug-1768 (compound/alter-nested primaries never have real slot/rusage availability checked — `seqjob_prepare_allocation()`/`parjob_prepare_allocation()` both no-op when `ctx->schedobj->tag != XCE_RESREQ_PRIMARY`) and bug-1769 (`sched_make_alter_allocation()` type-confuses `xce_compound_schedobj_t*` as `xce_sched_obj_t*`, a ~112-byte heap over-read, currently completely untested — the existing test believed to cover ALTER dispatch actually parses as PRIMARY per the resreq grammar). Neither is in this feature's scope to fix; both need dedicated follow-up.
+- (5.2) `core_begin_session_internal()` (`core_session.c`) is called from TWO structurally incompatible types sharing the same `xce_sched_core_t core` prefix: `driver.c` (`xce_scheduler_t*`) and `sim_loop.c` (`xce_sched_sim_driver_t*`, deliberately isolated, no cache field). Round 1's `__attribute__((weak))` cast-based approach caused every simulation session to silently corrupt `xce_sched_sim_driver_t.alt_events_file` (same byte offset as `xce_scheduler_t.unsatisfiable_cache`), invisible to ASAN (in-bounds) and to the whole test suite (no `test_sim_*.c` links `alloc_cache.c`). **Any future per-session hook that needs the real `xce_scheduler_t` (not just the shared `xce_sched_core_t` prefix) must use the driver-registered-callback pattern** (`xce_sched_core_t.cache_clear_cb`, set via `main.c` only in the driver-mode branch — `--sim` mode returns before reaching that registration — mirroring the existing `on_decision`/`xce_core_set_decision_cb()` precedent), never a cast of `core` back to `xce_scheduler_t*` inside code shared by both driver and sim paths.
+- (5.2) The real `cache_clear(xce_scheduler_t*)`→`xce_alloc_cache_clear_cb(xce_sched_core_t*)` adapter lives in `alloc_cache.c`, not `driver.c`, specifically because ~11 test files white-box `#include "../src/driver.c"` without linking `alloc_cache.c` — placing the adapter in `driver.c` would break their link. Any future adapter bridging a driver-only hook to a subsystem those tests don't link should follow this same placement rule (adapter lives with the subsystem it calls into, not with `driver.c`).