feat(xce-harness-component): xce-test-multihost builds/checks harness explicitly

run-multihost-verify.sh's XT default now points at
\$BUILD_DIR/harness/xce_test and fails fast (actionable "run
/xce-build harness" message) if missing. xce_test is the sole
mechanism (xtest()) by which the script's job-scenario functions
talk to the cluster, so a hard failure is correct, matching task
3.2's already-approved precedent.

SKILL.md's Prerequisites, env-var docs, and Dependencies sections
updated to match.

Verified live: sourced the script (main() guarded from running),
called precondition_checks() with the binary present and then moved
aside, confirmed the exact failure message, restored the binary
(checksum-verified intact).

Task 3.4 of xce-harness-component.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RLSNJhNoULvVcUhXEFafQC

.claude/skills/xce-test-multihost/SKILL.md

@@ -172,8 +172,10 @@ bash .claude/skills/xce-test-multihost/run-multihost-verify.sh [--keep-running]
 - Other overridable env vars: `$XCE_MULTIHOST_BUILD_DIR` (default
   `src/xce.build`), `$XCE_MULTIHOST_BIN` (controller),
   `$XCE_MULTIHOST_SCHED_BIN`, `$XCE_MULTIHOST_WORKER_BIN`,
-  `$XCE_MULTIHOST_XT` (`xce_test` -- REQUIRED as of task 4.2's job scenario,
-  a hard precondition failure if missing),
+  `$XCE_MULTIHOST_XT` (`xce_test` -- default `$XCE_MULTIHOST_BUILD_DIR/harness/xce_test`
+  since xce-harness-component task 3.1 decoupled it from the controller
+  build; REQUIRED as of task 4.2's job scenario, a hard precondition failure
+  if missing),
   `$XCE_MULTIHOST_PORT` (default `9991` -- single-node controller port),
   `$XCE_MULTIHOST_WORKER1`/`$XCE_MULTIHOST_WORKER2` (default
   `mh-worker-1`/`mh-worker-2`), `$XCE_MULTIHOST_WAITMS` (default `5000` --
@@ -198,21 +200,33 @@ verified safe for this exact pattern by the sibling skills above).
 
 ## Prerequisites
 
-Controller, scheduler, and worker components must already be built in the
-same meson build dir (default `src/xce.build`):
+Controller, scheduler, worker, and harness (`xce_test`) components must
+already be built in the same meson build dir (default `src/xce.build`).
+`xce_test` is decoupled from the controller build as of
+xce-harness-component task 3.1 -- it lives in the standalone `harness`
+component (`src/xce.build/harness/xce_test`) with its own `/xce-build
+harness` entry, and a controller-only build no longer produces it:
 
 ```
 /xce-build controller --incremental
 /xce-build scheduler --incremental
 /xce-build worker --incremental
+/xce-build harness --incremental
 ```
 
 (`--incremental` matters here specifically because a plain, non-incremental
 `/xce-build <component>` wipes the whole build dir before rebuilding just
 that one component -- see this project's own bug-1719 lesson -- so building
-controller/scheduler/worker one at a time into the same dir needs
+controller/scheduler/worker/harness one at a time into the same dir needs
 `--incremental` on every call after the first.)
 
+`run-multihost-verify.sh` itself also checks for all four binaries at
+startup (`precondition_checks()`) and fails fast with the exact `/xce-build`
+command to run if any is missing -- `xce_test` is this script's own driver
+client (every job submit/poll/verify step runs through it), so a missing
+harness binary is a hard `FAIL`, not a silently-skipped step or a raw
+spawn/connect error from the first `xtest()` call.
+
 ## Worker sizing (researched during task 4.1, wired in by task 4.2)
 
 `xcecd_virtual` has **no** command-line or config override for its reported
@@ -328,8 +342,8 @@ allocation decision, not a copy of the plain job's.
 - `xce-deploy-ha`'s `deploy-ha.sh` and its generated `start.sh` /
   `start-scheduler.sh` / `start-worker.sh` (single-controller-node path,
   two named worker instances against the same `--peers` config).
-- Controller, scheduler, and worker binaries in the same build dir (see
-  Prerequisites above).
+- Controller, scheduler, worker, and harness (`xce_test`) binaries in the
+  same build dir (see Prerequisites above).
 
 ## Requirements traceability
 

.claude/skills/xce-test-multihost/run-multihost-verify.sh

@@ -53,6 +53,17 @@
 # silently breaking the match (confirmed live: reproduced the exact false
 # negative, then confirmed ere_escape() fixes it, before wiring it in below).
 #
+# xce-harness-component task 3.4 (THIS revision): xce_test is no longer a
+# byproduct of the controller build (task 3.1 gave it its own standalone
+# `src/harness/` component and `/xce-build harness` entry) -- $XT's default
+# now points at `$BUILD_DIR/harness/xce_test`, and precondition_checks()
+# below fails with "run /xce-build harness" instead of "run /xce-build
+# controller" when it is missing. xce_test is this script's own driver
+# client (every job submit/poll/verify step in
+# run_multihost_job_scenario()/run_multihost_array_job_scenario() runs
+# through xtest()), so this stays a hard precondition failure, same as
+# controller/scheduler/worker above -- not a soft-skip.
+#
 # Requirements this task helps satisfy: 4.1 (controller ensures every
 # allocated host has launch info), 4.2 (worker starts tasks on every
 # allocated host), 4.3 (scheduler reports running via the existing
@@ -137,7 +148,12 @@ WORKER_BIN="${XCE_MULTIHOST_WORKER_BIN:-$BUILD_DIR/worker/xcecd_virtual}"
 # xce_test -- required by THIS task's submit_multihost_job()/jobinfo_of()
 # scripted sessions, following the exact idiom run-ha-soak.sh's
 # submit_worker_job()/jobinfo_of() already established.
-XT="${XCE_MULTIHOST_XT:-$BUILD_DIR/controller/xce_test}"
+# xce-harness-component task 3.1: xce_test now builds from the standalone
+# `src/harness/` component (its own `/xce-build harness` entry), not as a
+# side effect of `/xce-build controller` -- its default path here follows
+# that move. XCE_MULTIHOST_XT still lets a caller point at any build
+# dir/layout (e.g. a release build) explicitly.
+XT="${XCE_MULTIHOST_XT:-$BUILD_DIR/harness/xce_test}"
 
 # xce_test client-session tuning (run_multihost_job_scenario() below):
 #   WAITMS             wait-primary budget per xtest() session, ms. Single-
@@ -324,7 +340,15 @@ precondition_checks() {
     [ -x "$BINARY" ]    || fail "controller binary missing: $BINARY (run /xce-build controller)"
     [ -x "$SCHED_BIN" ] || fail "scheduler binary missing: $SCHED_BIN (run /xce-build scheduler)"
     [ -x "$WORKER_BIN" ] || fail "worker binary missing: $WORKER_BIN (run /xce-build worker)"
-    [ -x "$XT" ]         || fail "xce_test binary missing: $XT (run /xce-build controller) -- required by this task's job-scenario scripted sessions"
+    # xce-harness-component task 3.1 decoupled xce_test from the controller
+    # build -- it is this script's own driver client (every job
+    # submit/poll/verify step in run_multihost_job_scenario()/
+    # run_multihost_array_job_scenario() runs through xtest()), so a missing
+    # binary here is fatal, not soft-skippable like an optional dependency.
+    # Fail with the harness-specific build command instead of a raw
+    # spawn/connect error the first time xtest() tries to exec a
+    # nonexistent path.
+    [ -x "$XT" ]         || fail "xce_test binary missing: $XT (run /xce-build harness) -- required by this task's job-scenario scripted sessions"
     command -v nix >/dev/null 2>&1 || fail "nix not found on PATH -- required to resolve the deploy-environment bash and invoke deploy-ha.sh"
 }
 

.kiro/specs/xce-harness-component/tasks.md

@@ -49,7 +49,7 @@
   - _Requirements: 5.2, 5.4_
   - _Boundary: xce-deploy-ha skill_
   - _Depends: 3.1_
-- [ ] 3.4 (P) Update the multi-host verification workflow's harness prerequisite
+- [x] 3.4 (P) Update the multi-host verification workflow's harness prerequisite
   - Update `.claude/skills/xce-test-multihost/run-multihost-verify.sh` and `SKILL.md`
   - Observable: running the workflow from a build dir where only controller was built either builds the harness itself or reports a clear, actionable message
   - _Requirements: 5.3, 5.4_