test(scheduler-core): prove snapshot -EBUSY guard against a genuine active session (task 8.2)

Task 8.1's own -EBUSY guard test only proved rejection against a
synthetic single-thread session_active flag flip, with a comment
misattributing the technique to test_core_full_cycle.c's actually-
genuine-session precedent. Real gap, not busywork: xce_core_go()
publishes session_active via memory_order_release while the snapshot
guards read it via plain atomic_load() -- the synthetic same-thread
test never exercised any real cross-thread visibility of that flag.

Adds a genuine driver-driven session test (real picker thread + real
xce_core_go()) proving both save and load reject with -EBUSY during
the same active session, leaving state genuinely unchanged (sentinel
bytes for save; a distinguishable non-existent queue + unchanged
candidate count for load). Zero production changes -- task 8.1's
guards were already correct.

Closes out Group 8 (Core: Snapshot Serialization) entirely.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019dw5WnN3DXStaEaPYfACU1

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

@@ -207,11 +207,12 @@
   - _Depends: 7.3_
   - _Verified: 2026-08-04 — xce_sched_core_save_snapshot()/xce_sched_core_load_snapshot() implemented, split across snapshot.c/snapshot_wire.c/snapshot_load.c (800-line cap) along real logical boundaries (save/load/shared-helpers). New prng_seed field added to xce_sched_core_t (a genuine, well-investigated gap requirements.md/design.md both presupposed but no prior task added), seeded to 0, no PRNG consumer wired yet (documented, deliberate). Cross-core behavioral-equivalence Observable proven for real: two independent cores, each with its own registered picker, save on one/load into a freshly-initialized other, both driven through an identical real session (picker thread + decision drain), decision sequences asserted identical including a pinned spot-check (201-PEND/202-DISPATCH/203-PEND on both). -EBUSY/-EINVAL guards checked before any mutation, tested directly. Found and fixed two real bugs along the way: a missing prng_seed restore in this task's own new code, and a genuine, independently-traced-to-the-primitive-level guard bug in already-committed core.c (sched_get_user/sched_get_project treating key_find()'s expected -ENOENT as fatal, silently breaking get-or-create for any new name) that also affected the live production path (make_sched_job(), JOBS_PUSH) — narrowly scoped two-line fix, folded into this task's diff since only this task's own test path discovered it. 23/23 scheduler suite, full project build clean, review APPROVED first pass with every claim independently re-derived._
 
-- [ ] 8.2 Enforce the snapshot session-idle precondition
+- [x] 8.2 Enforce the snapshot session-idle precondition
   - Reject snapshot save/load calls made while a scheduling session is active, returning a distinct busy error rather than capturing or restoring torn state
   - Observable: attempting a snapshot save/load mid-session returns the busy error and leaves core state unchanged
   - _Requirements: 5.21_
   - _Depends: 8.1_
+  - _Verified: 2026-08-04 — task 8.1's own -EBUSY guard test only proved rejection against a synthetic single-thread session_active flag flip, and its comment misattributed the technique to test_core_full_cycle.c's actually-genuine-session precedent. Real gap, not busywork: xce_core_go() publishes session_active via memory_order_release while the snapshot guards read it via plain atomic_load() — the synthetic same-thread test never exercised any real cross-thread visibility of that flag. Added a genuine driver-driven session test (real picker thread + real xce_core_go()) proving both save and load reject with -EBUSY during the same active session, leaving state genuinely unchanged (sentinel bytes for save; distinguishable non-existent queue + unchanged candidate count for load) -- zero production changes, task 8.1's guards were already correct. 23/23 scheduler suite, 10/10 standalone flakiness-clean, review APPROVED first pass with an even sharper independent justification than the implementer's own. Closes out Group 8 (Core: Snapshot Serialization) entirely -- tasks 8.1-8.2 both complete._
 
 - [ ] 9. Core: Recording Control