test(scheduler-allocator-runner): characterize the parjob allocator (task 2.2)

15 tests drive the real parjob plugin through job_alloc_fn: success,
short shared, pending on hosts, mid-plan rollback, task counts 0/1,
missing inputs, six OOM steps with their debits, COMPOUND. Values the
runner changes on purpose sit in *_today tests.

Spec: the task showed that parjob records host reasons only for a bucket
search's first rejected candidate (the rejected host's request stays in
alloc_req.host and the host iterator pre-filters later hosts against
it). Decision: the runner clears alloc_req.host before every candidate,
for both allocators, so every short host gets its reasons (Req 3.6/3.7);
publishing reasons only when they change is a follow-up.

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

.kiro/specs/scheduler-allocator-runner/brief.md

@@ -117,40 +117,37 @@ goal that plugins no longer export `my_alloc()`).
   the seqjob OOM rollback of item 2 (Req 5 AC 5).
 - Divergences found in discovery that the design MUST resolve explicitly
   (they are not in the 2026-09-23 proposal):
-  1. **Decided 2026-09-29: the shared loop resets `ctx->alloc_req.host` to
-     empty before every bucket's host scan, for both allocators.** Background:
-     the per-host request used by the authoritative check is already rebuilt
-     for every candidate (`sched_try_alloc_on_host_()` -> `plugin_prepare_
-     allocation()` with `ctx->worker` set resets and refills it,
-     `plugin.c:639`). The per-bucket reset only affects the host iterator's
-     pre-filter (`host_iter.c:143`), which reads `alloc_req.host` before that
-     rebuild and silently skips hosts that fail. parjob already resets
-     (`parjob_allocator.c:479`), so its pre-filter does no resource filtering.
-     seqjob does not, so its pre-filter reads stale content: the job-level
-     `slots:1` for the first candidate, then the last rejected candidate's
-     full request. Resetting for seqjob too drops the stale-state dependency
-     and makes seqjob's pre-filter a resource no-op, like parjob's.
-     **Accepted consequence (intentional change):** host choice is unchanged
-     (a host the stale pre-filter rejects also fails the authoritative check
-     while the per-host request is host-independent; where a limit makes it
-     host-specific, the stale filter could wrongly skip a host, which the
-     reset fixes). But a pending seqjob job now gets a `PREASON_NO_RESOURCE`
-     host/resource reason from the authoritative check
-     (`resource.c:575`) for every host that passes the select filter and
-     lacks resources, not only for hosts the stale pre-filter let through.
-     `main_reason` is unaffected (host/resource reasons go to
-     `res_based_reasons`, `lib/preason.c:147`), so sim parity
-     (`sim_verify_pend_main_reason_eq`) is unaffected; no testsuite file
-     asserts `PREASON_*` host reasons (checked 2026-09-29).
-     Also accepted (found in design review): seqjob now runs the per-host
-     prepare fan-out on those hosts too, so the limit plugin may instantiate
-     a host limit's virtual resource and set its value on the worker earlier
-     than before (decision-neutral, lazy creation moves earlier), at the cost
-     of one prepare + check per short host per pending seqjob attempt.
-     Rejected: keeping the reset parjob-only (preserves the stale-state
-     quirk in shared code); staging each bucket's real request before the
-     scan so the pre-filter filters correctly (changes parjob's pend
-     reasons too; a separate follow-up if wanted).
+  1. **Decided 2026-09-29, revised 2026-09-30 (task 2.2 finding, user
+     decision): the runner clears `ctx->alloc_req.host` before every
+     candidate host (at the start of each bucket and after every rejected
+     candidate), for both allocators.** The per-host request used by the
+     authoritative check is rebuilt for every candidate anyway
+     (`plugin_prepare_allocation()` with `ctx->worker` set, `plugin.c:639`).
+     What the clear changes is the host iterator's pre-filter
+     (`host_iter.c:143`), which reads `alloc_req.host` before that rebuild and
+     silently skips hosts that fail. The 2026-09-29 plan (clear once per
+     bucket, as parjob does) turned out not to be enough: task 2.2's
+     characterization showed that a rejected candidate leaves its own request
+     behind, so parjob today records reasons only for the first rejected
+     candidate of a bucket search, and a once-per-bucket clear would give
+     seqjob the same partial reasons. With a clear before every candidate,
+     the pre-filter only applies the select expression, and every
+     selection-matching unclaimed short host gets its host/resource reasons,
+     in both allocators.
+     **Accepted consequences (intentional changes):** host choice, main
+     reason and decisions are unchanged (a host the stale pre-filter rejects
+     also fails the authoritative check while the per-host request is
+     host-independent; where a limit makes it host-specific, the stale filter
+     could wrongly skip a host, which the clear fixes). Pending jobs of both
+     allocators carry more host/resource reasons (`res_based_reasons`; not
+     `main_reason`, so the unsatisfiable cache and sim parity are unaffected).
+     The per-host prepare fan-out, including the limit plugin's lazy
+     host-limit instantiation, now runs on every short host (decision-neutral;
+     one prepare + check per short host per bucket search). Larger reason sets
+     are published and persisted; publishing reasons only when they change is
+     a follow-up (user, 2026-09-30), out of scope here.
+     Rejected: clearing once per bucket (partial reasons, still stale-state
+     dependent); keeping today's per-plugin behaviour.
   2. **Decided (requirements Req 5 AC 5):** seqjob builds its halloc directly
      from `ctx->alloc_req.host`; under the shared per-claim snapshot it gains
      a new `-ENOMEM` path, which goes through the same rollback