refactor(scheduler): move job-total limit computation to the limit plugin (bug-1790)

bug-1790's fix worked but put limit-plugin-specific logic in the wrong
place: seqjob_prepare_allocation()/parjob_prepare_allocation() (the
allocator plugins) unconditionally pre-computed a job-wide total for
every per-host resource in the job's rusages, "just in case" some
global limit needed one. That's the allocator plugins' concern leaking
into limit evaluation -- duplicated across both allocators (a third
would need it too), and wasteful (computed for resources no limit
actually references).

Moved ownership to sched_fill_limit_requests() (resource.c), called
once centrally from plugin_prepare_allocation() regardless of which
allocator ran:

- Removed calc_wanted_host_resval_job_totals() and its calls from both
  allocator plugins entirely.
- Added calc_wanted_job_total_resval() (resreq.c): an on-demand,
  single-resource lookup by name (no resmgr/key_find needed -- the
  caller already has the resolved xce_resource_t), computing the same
  job-wide-total formula only for the one (scope, underlying-resource)
  combination that needs it -- a GLOBAL limit on a per-host resource.
  The other three combinations are unchanged, still reading straight
  out of alloc_req.
- sched_fill_limit_requests() gains rusages/nhosts/nslots parameters
  back (a genuine need this time, not a re-added unused one).
  plugin.c's two call sites pass obj->u.primary->resreq->rusages
  (NULL for COMPOUND/ALTER, matching this codebase's existing scope
  convention for those tags) and ctx->nhosts/ctx->nslots.

All 23 testsuite link-satisfying stubs mechanically updated. Removed
the two allocator-level regression tests from the first pass (the
behavior they proved no longer exists by design); rewrote
test_fill_limit_requests_global_limit_on_host_resource_reads_host to
exercise the new on-demand path directly, nslots=3 to prove the job's
WHOLE total is used.

Full xce-build.sh all asan --incremental: 94/94 Ok, zero regressions.
Live re-verified against the same ug1_global_mem deploy/scenario:
byte-identical scheduler trace to the pre-refactor result, confirming
behavior-preserving.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GLfB5uWRdmx2WiZSFC7Fm7

src/scheduler/testsuite/test_sim_event_loop.c

@@ -145,10 +145,14 @@ void sched_worker_release_host_ctx(xce_sched_worker_t* w) { (void)w; }
  * linker dependency -- never invoked (this file never drives real
  * allocation, Req 5 AC 17 / this file's own header comment). */
 int sched_fill_limit_requests(xce_sched_alloc_req_t* alloc_req,
-                              xlist_t* limits)
+                              xlist_t* limits, xlist_t* rusages, int nhosts,
+                              int nslots)
 {
     (void)alloc_req;
     (void)limits;
+    (void)rusages;
+    (void)nhosts;
+    (void)nslots;
     return 0;
 }