feat(scheduler-allocation): rewrap seqjob allocation onto shared iterators

Restructure seqjob's my_alloc() as a trivial single-bucket case over the
shared xce_alloc_plan_iter_t/xce_host_iter_t contracts, replacing its own
inline worker-collection loop. Guard seqjob_prepare_allocation() to
maxTasks == 1, closing the fan-out gap for the parallel allocator's later
fix (Req 9.1, 4.6).

Export sched_try_alloc_on_host_() from resource.c (drop static/va_list,
plain (worker, sched, ctx) signature) so seqjob calls the real per-host
try-alloc leaf instead of a private duplicate; delete the now-dead
sched_try_alloc_on_hosts() variadic wrapper. Also wires host_iter.c into
the production scheduler_srcs list (task 2.1 only added it to testsuite).

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

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

@@ -39,7 +39,7 @@
   - _Boundary: alloc_cache.c_
 
 - [ ] 3. Core: seqjob allocator rewrap
-- [ ] 3.1 Rewrap seqjob's allocation onto the shared iterators
+- [x] 3.1 Rewrap seqjob's allocation onto the shared iterators
   - Restructure seqjob's allocation to use the shared plan-iterator and host-iterator contracts as a trivial single-bucket case, replacing its own inline worker-collection loop
   - Add a guard so seqjob's resource-request preparation only ever acts on single-task jobs, closing the fan-out gap this creates for the parallel allocator's later fix
   - Observable: seqjob's allocation still runs end to end using the shared iterators, with no allocator-specific worker-collection logic left in seqjob's own code
@@ -111,3 +111,5 @@
 - (1.4) When a new scheduler test needs a real function from `schedobj.c`/`core.c` (e.g. `sched_ctx_get_uncomit_host()`), white-box `#include` the real source file rather than privately reimplementing its logic in the test — established precedent: `test_picker_config_fields.c`'s `#include "../src/core.c"` + `#include "../src/schedobj.c"` pairing. A private reimplementation may pass today but silently drifts from the real function with nothing to catch it later.
 - (1.1) `xce_host_iter_t.next()` returns `xce_sched_worker_t*` (`sched/types.h`, carries `resvals`/`resInUse`/`host_eval_ctx`) — never `xce_worker_t*` (`xce/types.h`, the wire/config type `make_sched_worker()` converts FROM). requirements.md/design.md briefly had this wrong; fixed in commit `d1a3e158`. Task 2.1 (`host_iter.c`'s real implementation) and any later task touching worker candidates must use `xce_sched_worker_t*` from the start.
 - (1.2) `resreq_fp`'s ingest-time population lives in `core_build_primary_schedobj()` (`src/scheduler/src/core_events.c`), right after `primary->resreq = resreq;` — confirmed wired (min_tasks/max_tasks/select_node_id/rusage_hash all set from the parsed `xce_primary_resreq_t`) as of task 1.2 round 2, regression-covered by `test_core_build_primary_schedobj_populates_resreq_fp_from_parsed_resreq` in `src/scheduler/testsuite/test_core_events.c`. Round 1 added the type/field/hash machinery but never wired this call site, leaving `resreq_fp` permanently zeroed — do not rediscover this gap in a later task.
+- (3.1) The per-host try-alloc leaf (`sched_try_alloc_on_host_(xce_sched_worker_t* w, xce_scheduler_t* sched, xce_sched_ctx_t* ctx)` — select check → `sched_ctx_get_uncomit_host` → `plugin_prepare_allocation` → `sched_check_host_satisfy` → `xvec_idval(..., XVEC_ADD_FLOAT)`) now lives in `resource.c`, exported (non-static, non-variadic) and declared in `scheduler.h`. **Task 4.2 (parjob's per-bucket allocation loop) must call this exact shared function from its own bucket/host_iter loop, not write a second copy.** Round 1 of this task wrote a private ~70-line duplicate (`seqjob_try_alloc_on_host_`) because the original was `static`/`va_list`-based; round 2 fixed this by exporting the real function and deleting both the duplicate and the now-dead `sched_try_alloc_on_hosts()` variadic wrapper. This is the same reimplementation-vs-share risk task 1.4 hit in test code — here it recurred in production code, one task later.
+- (3.1) `src/scheduler/meson.build`'s production `scheduler_srcs` list was missing `host_iter.c` since task 2.1 only wired that file into `testsuite/meson.build` — invisible until task 3.1 became the first *production* caller of `host_iter.c`'s functions (a shared .c file with zero production callers builds/links fine without being in `scheduler_srcs` at all, so task 2.1's own green build/test run couldn't have caught this). Any future task adding a new shared scheduler `.c` file should double check it lands in `scheduler_srcs`, not only `testsuite/meson.build`, even if nothing calls it yet.