ci(observability-map): stop the nightly corpus depending on the paths filter

`needs: changes` carries an implicit success() that outranks the event test in
the `if`, so a failed or skipped filter job silently skipped the nightly
mutation corpus and the tree-drift scan stopped without saying so. Adding a
status-check function to the `if` drops the implicit success() and lets the
event test decide alone.

The filter job now runs only for pull requests, the sole path that reads its
output. Pull request gating is unchanged.

Author: 1stvamp

.github/workflows/observability-map.yml

@@ -26,6 +26,10 @@ jobs:
   # evaluates it once per workflow. This narrows it again for the corpus job alone.
   changes:
     name: 🔍 Which paths moved
+    # Only the pull request path reads this job's output. On a schedule the action has no base to
+    # diff, warns that `before` is missing and reports the files in the last commit on main, which
+    # nothing then consults. Skipping it there keeps the nightly off a job it does not need.
+    if: github.event_name == 'pull_request'
     runs-on: warp-ubuntu-latest-x64-2x
     outputs:
       package: ${{ steps.filter.outputs.package }}
@@ -63,7 +67,20 @@ jobs:
   mutation-corpus:
     name: 🧬 Mutation corpus
     needs: changes
-    if: github.event_name != 'pull_request' || needs.changes.outputs.package == 'true'
+    # `!cancelled()` is here for the nightly, not for tidiness. `needs` carries an implicit
+    # success() on the job it names, and that implicit test outranks the `||` below: with a plain
+    # condition, a `changes` job that failed or was skipped skips this one, so the nightly would
+    # stop scanning for tree drift and report nothing about having stopped. A status-check function
+    # in the `if` is what drops the implicit success(), so the event test below decides alone.
+    # `!cancelled()` rather than `always()` because `cancel-in-progress` above is a real path and a
+    # superseded run should not finish this job.
+    #
+    # Pull request behaviour is deliberately unchanged: on a PR a failed `changes` leaves
+    # `needs.changes.outputs.package` empty, so the corpus still skips. The nightly is the backstop
+    # for that, which is the same trade the paths gate already makes for routes-only pull requests.
+    if: >-
+      !cancelled() &&
+      (github.event_name != 'pull_request' || needs.changes.outputs.package == 'true')
     runs-on: warp-ubuntu-latest-x64-4x
     steps:
       - name: ⬇️ Checkout repo