ci(1874): upload the per-iteration evidence that was being silently dropped

`.tmp` is a hidden directory and upload-artifact skips those unless told to
include them, so every per-iteration log this loop has ever kept was discarded —
including on the dispatch that motivated capturing slow passes in the first
place, where the summary arrived with its cadence excerpt and none of the logs it
points at. Same input the shared diagnostics action sets, for the same reason.

Author: thymikee

.github/workflows/1874-diagnose.yml

@@ -126,6 +126,11 @@ jobs:
         uses: actions/upload-artifact@v4
         with:
           name: stall-evidence-${{ matrix.arch }}-${{ matrix.mode }}
+          # `.tmp` is a hidden directory, and upload-artifact skips those unless told otherwise —
+          # so every per-iteration log this loop has ever kept was silently dropped, including on
+          # the run that motivated the slow-pass capture. Same input the shared diagnostics action
+          # sets, for the same reason.
+          include-hidden-files: true
           path: |
             stall-summary.txt
             .tmp/stall-evidence-*.log