feat: port GTK version fallbacks, helpgen, bench modes, CLI leftovers

- apprt-gtk: Ghostty's runtime+build-time version gating (at_least)
  with cargo features adw_1_3/adw_1_4/adw_1_5/gtk_4_16 (default on) and
  the fallbacks for older libraries: MessageDialog (adw < 1.5),
  clipboard dialog without SwitchRow, pre-Banner debug warning and
  child-exited widgets, the old about dialog, GTK 4.16 CSS error source
  bytes. The minimum stays GTK 4.14 / libadwaita 1.5 (Ghostty's floor).
  The root build links with -z lazy so version-gated symbols don't keep
  older libraries from loading.
- ghostty-helpgen (port of helpgen.zig) generates CLI action help and
  keybinding action docs from doc comments at build time, replacing the
  checked-in help/*.txt; +help and +list-actions --docs stay
  byte-identical to Ghostty 1.3.1.
- ghostty-bench: libc wcwidth mode and separate simd/table and
  uucode/table code paths like Zig.
- ghostty-cli: Windows branches of edit-config, list-colors,
  list-keybinds and boo; ssh-cache DiskCache flock locking (no-op on
  Windows, as in Zig) with an exclusivity test.

Test Plan:
- cargo test --workspace
- diff <(cargo run -q -- +list-actions --docs) \
  <(ghostty +list-actions --docs)

docs/rewrite/notes/compat-GC-bench.md

@@ -0,0 +1,18 @@
+# ghostty-bench: CodepointWidth / IsSymbol (compat-GC)
+
+- `codepoint-width` modes (CLI as Zig: noop|wcwidth|simd|table):
+  - `wcwidth`: real libc `wcwidth` via a local `extern "C"` (no libc crate), in
+    a module with `#[allow(unsafe_code)]` + SAFETY comment. Zig does not call
+    `setlocale`, so neither do we (C locale). Non-unix: warning no-op like Zig on Windows.
+  - `simd`: new `src/simd_width.rs`, scalar port of `simd/codepoint_width.cpp`
+    (same range tables and precedence; vector lanes replaced by a sequential scan).
+  - `table`: generated two-stage table with the `<= 0xFF => 1` shortcut.
+- `is-symbol` modes: `uucode` (default) = `is_symbol_computed`, computed from
+  private-use + block ranges (`computeIsSymbol`); `table` = generated symbols table.
+  Test verifies equality with the table over all codepoints.
+- Cargo.toml: `[lints]` no longer `workspace = true` (workspace `forbid` cannot be
+  locally allowed); `unsafe_code = "deny"` and the clippy set is copied. Keep in sync.
+- Deviation: Zig's `simd` falls back to uucode when built without SIMD; there is no
+  separate uucode width mode in the CLI (none in Zig), so none added.
+- Tests: all modes run on a sample file; computed vs table symbols; simd vs table
+  samples; wcwidth ASCII. Zig `test CodepointWidth/IsSymbol` (run once, no data) covered by the run tests.