drm: close the round-7 review findings

- the renumbering probe in the DrmDisplaysChanged handler now reads the
  pushed list at wire_idx, the slot our monitor held in the service's
  index space, instead of at the index the client chose. the pushed
  list shares the handshake list's construction, so probing the client
  index compared two different index spaces whenever a wake or hotplug
  had renumbered entries - tearing down a healthy stream or missing a
  real renumbering.
- both message-body reads (cpu frame, cursor pixels) now run under a
  deadline. only the header read re-checked `stop`, so a producer dying
  between a header and its body pinned the receive thread forever and
  every rebuild leaked a thread plus its render context.
- the drm cursor cache gets a size ceiling (drm ids are derived from
  the shape's content, so an animated pointer minted a new key per
  shape and the map grew for the life of the service; x11 ids come
  from a small serial set, so the ceiling is gated and the stock build
  is untouched).
- has_non_drm_backed_display reads a two-scalar accessor instead of
  cloning and geometry-augmenting the whole display list on every
  cursor tick.
- the libdrmtap pin validation moved out of import time into
  build_libdrmtap_so(), so leftover DRMTAP_* environment variables or a
  malformed sha cannot fail a stock build that never touches libdrmtap.
- reworded a workflow comment whose literal expression marker broke
  actionlint.

.github/workflows/drm-capture.yml

@@ -293,7 +293,9 @@ jobs:
           # `[[ ... ]] && cmd` as the last line makes the STEP fail once FLUTTER_VERSION moves off
           # the pinned value, because the failed test becomes the script's exit status. An explicit
           # if/else skips instead. Reading the values from the environment rather than interpolating
-          # ${{ }} into the script also keeps this off zizmor's template-injection list.
+          # github expressions into the script also keeps this off zizmor's template-injection list.
+          # (spelled out in prose: a literal expression marker here, even in a comment, is parsed by
+          # actionlint and breaks workflow linting.)
           if [[ "$FLUTTER_VERSION" == "3.24.5" ]]; then
             git apply "$GITHUB_WORKSPACE/.github/patches/flutter_3.24.4_dropdown_menu_enableFilter.diff"
           else