test(hook): assert the seam precondition and self-diagnose CI failures

The conflict regression went red on six CI legs with an unhelpful
message: every probe produced empty stderr, which reads as "the notice
is missing" when it actually means no conflict was ever raised. Three
changes, all diagnosis rather than tolerance:

- Assert the binary carries the hook-client seam up front, failing with
  the rebuild hint, exactly as tests/test_worker_watchdog.sh does for
  the crash-orphan probe. A seam-less binary can never raise the
  conflict, and that must be said plainly rather than inferred from a
  silent backstop expiry.
- Drop the `seq`-derived fingerprint for a literal 64-char constant
  (length-checked), removing a dependency on a tool whose presence
  varies across the MSYS2/macOS/Linux legs.
- On backstop expiry, dump stdout, the forced fingerprint and its
  length, and daemon status, so the next run names the cause instead of
  requiring another round trip.

Still green locally (seam-bearing binary): the conflict is observed and
the systemMessage asserted.

Signed-off-by: Martin Vogel <martin.vogel.tech@gmail.com>

tests/test_hook_conflict_notice.sh

@@ -16,6 +16,17 @@ if [[ ! -x "${BINARY}" ]]; then
   exit 2
 fi
 
+# PRECONDITION, not a skip (same doctrine as tests/test_worker_watchdog.sh): this
+# test drives the hook-client build seam, which is compiled out of ordinary
+# builds. Against a seam-less binary no conflict can ever occur and the failure
+# would read as "the notice is missing" instead of "the capability is absent".
+if ! LC_ALL=C grep -a -q -F 'CBM_TEST_HOOK_CLIENT_BUILD' "${BINARY}"; then
+  echo "binary lacks the hook-client build seam: ${BINARY}" >&2
+  echo "  rebuild it with:  make -f Makefile.cbm cbm TEST_SEAMS=1" >&2
+  echo "  or run this test through scripts/test.sh, which does that for you." >&2
+  exit 2
+fi
+
 tmpdir="$(mktemp -d)"
 cleanup() {
   CBM_CACHE_DIR="${tmpdir}/cache" "${BINARY}" daemon stop >/dev/null 2>&1 || true
@@ -29,7 +40,11 @@ if ! CBM_CACHE_DIR="${tmpdir}/cache" "${BINARY}" daemon start >"${tmpdir}/daemon
   exit 2
 fi
 payload='{"session_id":"probe","hook_event_name":"PreToolUse","tool_name":"Grep","tool_input":{"pattern":"x","path":"/tmp"},"cwd":"/tmp"}'
-forced_build="$(printf 'f%.0s' $(seq 1 64))"
+forced_build="ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff"
+if [[ ${#forced_build} -ne 64 ]]; then
+  echo "internal: forced fingerprint must be 64 hex chars, got ${#forced_build}" >&2
+  exit 2
+fi
 
 # The daemon's cohort join completes asynchronously after `daemon start`
 # returns. Wait for the STATE the assertion needs - a probe that observes the
@@ -62,8 +77,14 @@ if ! grep -q "conflicting CBM process" "${tmpdir}/probe.err"; then
   echo "never became active, or this is not a TEST_SEAMS=1 binary" >&2
   echo "--- daemon-start.log ---" >&2
   cat "${tmpdir}/daemon-start.log" >&2
-  echo "--- last probe stderr ---" >&2
+  echo "--- last probe stderr (empty means no conflict was raised) ---" >&2
   cat "${tmpdir}/probe.err" >&2
+  echo "--- last probe stdout ---" >&2
+  printf '%s\n' "${out}" >&2
+  echo "--- diagnosis ---" >&2
+  echo "forced_build=${forced_build} (len ${#forced_build})" >&2
+  echo "seam present in binary: yes (asserted above)" >&2
+  CBM_CACHE_DIR="${tmpdir}/cache" "${BINARY}" daemon status >&2 2>&1 || true
   exit 2
 fi
 if ! printf '%s' "${out}" | grep -q "systemMessage"; then