fix(install): escape sed replacements, use mktemp, and make render failures fatal

Address CodeRabbit findings on install_service.sh, install_web_service.sh and
install_wifi_monitor.sh:

- Values interpolated into each script's sed expression (project root path,
  username) were not escaped, so a value containing &, \ or the | delimiter
  would corrupt the rendered systemd unit. Add a shared
  sed_escape_replacement() helper in the new scripts/install/lib_systemd_render.sh
  (sourced by all three scripts) and apply it to every sed replacement.
- install_service.sh rendered the main and web units to the predictable path
  /tmp/ledmatrix.service.tmp before installing them -- a symlink/TOCTOU race
  (CWE-377). Use mktemp for both, with a trap to clean up on exit.
- install_service.sh treated a missing template as a mere warning and then
  checked only whether a unit already existed at the destination before
  enabling/starting it, so a render failure could silently fall back to
  enabling a stale, previously-installed unit. Both unit blocks now exit
  non-zero on a missing template or a failed render.

Also rename the ambiguous loop variable `l` to `line` in
test/test_systemd_unit_drift.py (Ruff E741); ruff isn't wired into any CI
workflow in this repo today, so this isn't currently CI-blocking, but the
rename is trivial and correct regardless.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S3bPMESe2TfrGvbs1ef9c5

scripts/install/install_web_service.sh

@@ -17,6 +17,9 @@ fi
 # Determine the Project Root Directory (parent of scripts/install/)
 PROJECT_ROOT_DIR=$(cd "$(dirname "${BASH_SOURCE[0]}")/../.." && pwd)
 
+# shellcheck source=scripts/install/lib_systemd_render.sh
+source "$PROJECT_ROOT_DIR/scripts/install/lib_systemd_render.sh"
+
 echo "Installing for user: $ACTUAL_USER"
 echo "Project root directory: $PROJECT_ROOT_DIR"
 
@@ -38,7 +41,9 @@ if [ ! -f "$TEMPLATE" ]; then
 fi
 
 echo "Writing service file to /etc/systemd/system/ledmatrix-web.service"
-sed "s|__PROJECT_ROOT_DIR__|$PROJECT_ROOT_DIR|g; s|__USER__|$ACTUAL_USER|g" \
+ESCAPED_PROJECT_ROOT_DIR=$(sed_escape_replacement "$PROJECT_ROOT_DIR")
+ESCAPED_ACTUAL_USER=$(sed_escape_replacement "$ACTUAL_USER")
+sed "s|__PROJECT_ROOT_DIR__|$ESCAPED_PROJECT_ROOT_DIR|g; s|__USER__|$ESCAPED_ACTUAL_USER|g" \
     "$TEMPLATE" > /etc/systemd/system/ledmatrix-web.service
 
 # Ensure cache directory exists with proper permissions