ci: a truncated PR file list selects every lane

The pulls/files API lists at most 3000 files and says nothing when it stops,
and the lane decision is only as complete as that list: a cut list can drop
exactly the file that needed a lane.

select-lanes.sh takes --expect-files N (the PR's changed_files; batch rows
take "expect_files") and decides full with a file_list_truncated reason when
the list has fewer distinct paths than N (a complete list never does: renames
only add their old path), when N is at or over the 3000-file cap, or when the
list itself reaches the cap. The tier still classifies what was listed. An
empty or non-numeric count is a usage error, so a workflow that loses the
count fails loudly instead of skipping the check.

pr.yml passes github.event.pull_request.changed_files. The selector contract
covers short, complete, rename-inflated, at-cap and just-under-cap lists and
the github-output and batch channels; the wiring contract requires the count
to be passed.

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

.github/workflows/pr.yml

@@ -108,10 +108,14 @@ jobs:
         with:
           persist-credentials: false
 
+      # The pulls/files API lists at most 3000 files: the PR's own count lets
+      # the selector notice a cut list and select everything instead.
       - name: Select lanes
         id: select
+        env:
+          CHANGED_FILES: ${{ github.event.pull_request.changed_files }}
         run: |
-          scripts/ci/select-lanes.sh --format github-output "$RUNNER_TEMP/changed-files.txt" > "$RUNNER_TEMP/selection.txt"
+          scripts/ci/select-lanes.sh --format github-output --expect-files "$CHANGED_FILES" "$RUNNER_TEMP/changed-files.txt" > "$RUNNER_TEMP/selection.txt"
           cat "$RUNNER_TEMP/selection.txt"
           cat "$RUNNER_TEMP/selection.txt" >> "$GITHUB_OUTPUT"
           # pr-smoke's legs, each tagged with its lane (kept in the job name,