ci: fail the mutation floors step, and scale nightly property runs by a factor

The floors step piped scripts/mutation-floors.mjs into tee without
pipefail, so it took tee's exit status and a module under its floor could
never fail the run. It now runs under bash with pipefail, as do the other
steps that pipe into tee, and a guard reads every workflow and fails on a
tee pipe without it. The other piped steps were checked: each either
validates what the pipe produced or pipes a command that cannot fail.

The nightly runs set FC_NUM_RUNS, an absolute count that replaced every
property's own: the database backup round trip, which states twelve cases
for its cost, ran a hundred times against a timeout sized for twelve.
FC_RUNS_FACTOR now multiplies each property's count (five for the unit
suite, four for integration), propertyTimeout grows a test's timeout by
the same factor, and the backup round trip uses it.

.github/workflows/mutation.yml

@@ -35,8 +35,13 @@ jobs:
       - name: Run Stryker
         run: pnpm exec stryker run --concurrency 4
 
+      # Without pipefail the step takes the exit status of `tee`, which is
+      # always 0, so a module under its floor could never fail the run.
       - name: Check the per-module floors
-        run: node scripts/mutation-floors.mjs | tee -a "$GITHUB_STEP_SUMMARY"
+        shell: bash
+        run: |
+          set -o pipefail
+          node scripts/mutation-floors.mjs | tee -a "$GITHUB_STEP_SUMMARY"
 
       - name: Upload the report
         if: always()