fix(gen-keybinds): match the section dash as a literal so headers survive LC_ALL=C

The section rule was `/^[[:space:]]*\/\/[[:space:]]*─+ (.+) ─+…$/`, a quantifier on a
multibyte character. Under LC_ALL=C — which is what cron, systemd user units and agent
shells get, regardless of LANG — gawk matches byte-wise, so `─+` means "the last byte of
U+2500, repeated" and never matches. Measured on the live keybinds.kdl: the old script
emitted 0 section headers instead of 12, all 120 binding rows intact, exit status 0. A
silent one-third-of-the-page loss, invisible to any caller that only checks the status.

The dash is now matched and stripped as a literal (`gsub(/─/, "")`), which is byte-safe
in every locale, and a comment must be dash-decorated on both sides to open a section, so
plain comments are still ignored. A test pins the C-locale case explicitly, because the
box's own terminal has always been UTF-8 and could never have caught this.

The committed artifact omp-docs/tools/niri/keybindings.md is unaffected (12 headers,
120 rows — it was generated from a UTF-8 shell); output is now identical either way.

tests/gen-keybinds.bats

@@ -48,6 +48,16 @@ teardown() {
   [[ "$output" != *"a plain comment"* ]]
 }
 
+@test "section headers survive a C locale, where awk reads bytes instead of characters" {
+  # Cron, systemd units and agent shells get LC_ALL=C while Frank's terminal does not.
+  # A matcher that puts a quantifier on a multibyte character then loses EVERY section
+  # header and prints a sheet with no sections at all — silently, exit status 0.
+  run env LC_ALL=C "$GEN" "$TMP/keybinds.kdl"
+  [[ "$output" == *"## Applications"* ]]
+  [[ "$output" == *"## Screenshots"* ]]
+  [[ "$output" != *"a plain comment"* ]]
+}
+
 @test "row count matches bind count" {
   run "$GEN" "$TMP/keybinds.kdl"
   local rows