fix(scheduler): cache hits keep the full pend reason; sim uses the cache (bug-1932)

The session-level unsatisfiable-resreq cache stored only the exhausted
attempt's main reason (an int). The allocators record shortfalls as
per-host/per-resource entries and leave main_reason at 0, so every cache
hit in the live scheduler pended its job with an empty reason. The
simulator registered no cache at all (its table lived on xce_scheduler_t),
so it searched every identical pending job again and its traces differed
from production.

- The cache table moves to xce_sched_core_t; cache_lookup/insert/clear and
  the hook adapters take the core, with no cast to a driver struct.
  sched_init() builds it, cache_clear() rebuilds it per session,
  xce_sched_core_fini() destroys it.
- A cache entry is a deep copy of the attempt's full xce_job_preason_t
  plus the job that proved the request unsatisfiable (core->alloc_jobid,
  set by sched_allocator()). A hit moves a copy into ctx->reason; a miss
  leaves ctx->reason untouched.
- main.c passes the same three adapters to the simulator through new
  sched_sim_args_t fields; sched_sim_driver_start() sets them on the sim
  core (plain field writes: white-box tests include sim.c alone).
- Decision trace: a hit renders "no plan generated: skipped, same request
  as <job> proved unsatisfiable earlier this session" (new
  XCE_TRACE_NOPLAN_KNOWN_UNSAT via sched_trace_known_unsat()); the outcome
  shows the restored reasons.
- sim_verify keeps its main-only PEND compare (old recordings carry
  main-only hits); comments and the scheduler-sim-live-allocation design
  note updated.

Tests: full-reason assertions in the cache dispatch tests (RED before:
"a cache hit must restore the original attempt's FULL pend reason"),
test_alloc_cache.c ported to the new API (deep copy, clear frees, -EEXIST
frees), a sim-registered clear test, a renderer test, and a sim e2e
scenario with two identical unsatisfiable jobs (204, 205) where 205 is
skipped with 204's full reasons (RED before: no skip line).

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

.kiro/specs/scheduler-sim-live-allocation/design.md

@@ -35,6 +35,8 @@
 - bug-1827 (the `core_clock.c` isolation-test coverage gap) — unrelated, tracked separately.
 - Any change to `.replay` file format, recording/capture (`recording.c`/`recording_events.c`), or snapshot format (`snapshot_wire.c`/`snapshot_load.c`) — confirmed field-complete and unaffected by the audit (dimension 4).
 - Giving simulation mode a working `unsatisfiable_cache` implementation of its own.
+
+> **Superseded (2026-10-09, bug-1932):** the two cache non-goals above no longer hold. The cache table moved from `xce_scheduler_t` onto `xce_sched_core_t`, the simulator registers the same three hooks as normal mode (passed in through `sched_sim_args_t`), and a hit now restores the original attempt's full pend reason instead of only its main reason. The seam itself is unchanged in role: no hook casts the core to a driver struct.
 - Simulation-mode thread lifecycle/teardown (`sched_fini()`-equivalent for sim mode does not exist today and this spec does not add one).
 
 ### Allowed Dependencies