test(fleet): bound stale heartbeat observation (#1043)

docs/scratchpads/1043-pane-git-identity.md

@@ -192,6 +192,37 @@ This is the third GNU-host/Alpine-CI portability defect in the lane: GNU `grep`
 - Workflow definition population: **1/3 pull-request-eligible** (`ci.yml` of `ci.yml`, `ci-image.yml`, `publish.yml`).
 - Do not re-litigate the settled 311/312 populations; both are valid for their separately named Tess6 and CRUD-core7 sets.
 
+## Round 4 — bound stale-marker observation
+
+### Objective and plan
+
+- Make the heartbeat assertion discriminate an initially stale native marker from a fresh marker without changing the production staleness threshold or shortening the polling window.
+- Freeze only the sidecar's numeric observation clock during the stale-fixture arm so elapsed assertion time cannot turn a fresh mutant stale.
+- Prove two independent mutants RED: disable production stale-marker detection while retaining the stale fixture; replace the stale fixture with a fresh marker. Restore the tree and prove GREEN in the canonical Alpine image.
+- Re-derive the changed-path inventory, run applicable quality and independent review gates, commit with environment-only author/committer identity, queue-guard, push once, verify provider attribution using curl stdin config, and stop without CI polling.
+- Working budget: 8K tokens. Scope is the launcher test and its scratchpad evidence; production launcher behavior remains unchanged.
+
+### Root cause and bounded observation
+
+The 30 × 0.1-second assertion window overlaps the production `now - marker > interval * 2 + 1` threshold at interval 1. Depending on second boundaries and load, a fresh marker can age past the threshold before the assertion ends. A focused pre-fix fresh-mutant attempt returned RED while Review 101's full-suite run returned GREEN; the differing result is itself timing dependence, not a discriminating assertion.
+
+The test now supplies a fixed numeric epoch only to the stale-fixture sidecar. Its real marker mtime is still read from the filesystem, but assertion runtime cannot advance `now`. Date formatting still delegates to the image's real `/bin/date`. Neither the production threshold nor the 30 × 0.1-second polling window changed.
+
+### Two-mutant RED / restored GREEN
+
+All three runs used `git.mosaicstack.dev/mosaicstack/stack/ci-base:latest`:
+
+1. **Stale-detection mutant RED:** replaced only the production stale-age predicate with `false` while retaining the fixed stale marker; suite exit 1 with `FAIL: heartbeat sidecar did not resume after native marker became stale or absent`.
+2. **Fresh-marker mutant RED:** replaced only `touch -t 200001010000.00` with fresh `touch`; suite exit 1 with the same failed stale-resumption assertion. The fixed observation epoch kept the mutant fresh throughout all 30 polls.
+3. **Restored tree GREEN:** suite exit 0 with `ok - start-agent-session generated environment boundary`.
+
+### Re-derived inventory
+
+- Round-4 delta: **2/2 files** — launcher test plus task scratchpad; production launcher delta is empty.
+- Full PR inventory against `origin/main`: **19/19 paths**; Round 4 adds no path.
+- Production stale threshold remains `now - marker > iv * 2 + 1`; assertion polling remains 30 × 0.1 seconds.
+- Review 101's confirmed enumeration/workflow/CI and attribution evidence is accepted without re-polling or re-derivation.
+
 ## Residual risk
 
 - Landing on `main` does not update the currently installed host launcher. Host framework installation/reseed and Jarvis live-seat validation are separate downstream events.

packages/mosaic/framework/tools/fleet/test-start-agent-session.sh

@@ -62,6 +62,19 @@ env -0 > "${MOSAIC_HOME:?}/fleet/pane-environment"
 SHIM
 chmod +x "$FAKE_BIN/mosaic"
 
+# Freeze numeric epoch reads only when a test arm supplies an observation bound.
+# Formatting reads still use the real BusyBox/POSIX date implementation.
+cat > "$FAKE_BIN/date" <<'SHIM'
+#!/usr/bin/env bash
+set -euo pipefail
+if [ -n "${MOSAIC_TEST_FIXED_EPOCH:-}" ] && [ "${1:-}" = '+%s' ]; then
+  printf '%s\n' "$MOSAIC_TEST_FIXED_EPOCH"
+  exit 0
+fi
+exec /bin/date "$@"
+SHIM
+chmod +x "$FAKE_BIN/date"
+
 write_generated() {
   local home="$1"
   local agent="$2"
@@ -89,6 +102,7 @@ run_start() {
   local agent="$2"
   HOME="$home" PATH="$FAKE_BIN:$PATH" MOSAIC_TEST_TMUX_CALLS="$TMUX_CALLS" \
     MOSAIC_TEST_PANE_PID="${MOSAIC_TEST_PANE_PID:-}" \
+    MOSAIC_TEST_FIXED_EPOCH="${MOSAIC_TEST_FIXED_EPOCH:-}" \
     MOSAIC_TEST_HOME="$home" \
     MOSAIC_TEST_FLEET_OWNER=123e4567-e89b-12d3-a456-426614174000 \
     MOSAIC_HOME="$home" "$START" "$agent"
@@ -403,7 +417,11 @@ write_heartbeat_local "$HOME_NATIVE_STALE" "coder-native-stale"
 STALE_HB="$HOME_NATIVE_STALE/run/coder-native-stale.hb"
 printf 'ts=native\npid=1\nstatus=busy\nmodel=stale-model\n' > "$STALE_HB"
 touch -t 200001010000.00 "$STALE_HB.native"
-MOSAIC_TEST_PANE_PID=$$ run_start "$HOME_NATIVE_STALE" coder-native-stale
+# Hold the sidecar's observation epoch constant: assertion runtime must not age
+# a fresh-marker mutant into the stale state that this fixture must distinguish.
+STALE_OBSERVATION_EPOCH=$(date +%s)
+MOSAIC_TEST_FIXED_EPOCH="$STALE_OBSERVATION_EPOCH" \
+  MOSAIC_TEST_PANE_PID=$$ run_start "$HOME_NATIVE_STALE" coder-native-stale
 wait_for_sidecar_status "$STALE_HB"
 
 HOME_NATIVE_ABSENT="$ROOT/native-absent"