feat(vt): port Ghostty's VT parsing layer

Ports the parsing half of ZIG/src/terminal into the ghostty-vt crate:
- Parser state machine with a compile-time generated parse table and
  the UTF-8 decoder
- OSC parser incl. all sub-parsers (titles, colors, clipboard, hyperlinks,
  semantic prompts/OSC 133, OSC 9 / ConEmu, iTerm2, kitty clipboard,
  color and text-sizing protocols, context signals)
- CSI, SGR, modes table with save/restore, charsets, ansi, device status,
  mouse shapes, APC, DCS (incl. XTGETTCAP), tmux control-mode parser
- color palette / X11 color names, kitty color and graphics command parser

Deviations from Zig (documented in docs/rewrite/notes/ghostty-vt-W*.md):
parser actions own their payloads instead of borrowing parser buffers;
OSC strings are Vec<u8>; non-exhaustive Zig enums gain an Other(raw)
variant; tmux regexes replaced by equivalent hand-written matchers.

Known follow-up: semantic_prompt.rs carries local copies of the printf-q
and percent decoders that belong in ghostty-os.

Test Plan:
- cargo test -p ghostty-vt: 395 passed, 1 ignored (skipped in Zig too)
- cargo clippy -p ghostty-vt --all-targets -- -D warnings: clean

crates/ghostty-vt/src/charsets.rs

@@ -0,0 +1,121 @@
+//! Character sets (ZIG/src/terminal/charsets.zig).
+
+/// The available charset slots for a terminal.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub enum Slots {
+    G0,
+    G1,
+    G2,
+    G3,
+}
+
+/// The name of the active slots.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub enum ActiveSlot {
+    GL,
+    GR,
+}
+
+/// The list of supported character sets and their associated tables.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub enum Charset {
+    Utf8,
+    Ascii,
+    British,
+    DecSpecial,
+}
+
+/// Our table length is 256 so we can contain all ASCII chars.
+pub const TABLE_LEN: usize = 256;
+
+/// The table for the given charset. The returned table is guaranteed to be
+/// 256 entries that can be used to map ASCII to the given charset.
+///
+/// # Panics
+/// Panics for `Charset::Utf8`: utf8 is not a table, callers should
+/// double-check if the charset is utf8 and NOT use tables.
+pub fn table(set: Charset) -> &'static [u16; TABLE_LEN] {
+    match set {
+        Charset::British => &BRITISH,
+        Charset::DecSpecial => &DEC_SPECIAL,
+        Charset::Utf8 => unreachable!("utf8 has no table"),
+        Charset::Ascii => &ASCII,
+    }
+}
+
+/// Creates a table that maps ASCII to ASCII as a getting started point.
+const fn init_table() -> [u16; TABLE_LEN] {
+    let mut result = [0u16; TABLE_LEN];
+    let mut i = 0;
+    while i < TABLE_LEN {
+        result[i] = i as u16;
+        i += 1;
+    }
+    result
+}
+
+/// Just a basic c => c ascii table
+static ASCII: [u16; TABLE_LEN] = init_table();
+
+/// https://vt100.net/docs/vt220-rm/chapter2.html
+static BRITISH: [u16; TABLE_LEN] = {
+    let mut t = init_table();
+    t[0x23] = 0x00a3;
+    t
+};
+
+/// https://en.wikipedia.org/wiki/DEC_Special_Graphics
+static DEC_SPECIAL: [u16; TABLE_LEN] = {
+    let mut t = init_table();
+    t[0x60] = 0x25C6;
+    t[0x61] = 0x2592;
+    t[0x62] = 0x2409;
+    t[0x63] = 0x240C;
+    t[0x64] = 0x240D;
+    t[0x65] = 0x240A;
+    t[0x66] = 0x00B0;
+    t[0x67] = 0x00B1;
+    t[0x68] = 0x2424;
+    t[0x69] = 0x240B;
+    t[0x6a] = 0x2518;
+    t[0x6b] = 0x2510;
+    t[0x6c] = 0x250C;
+    t[0x6d] = 0x2514;
+    t[0x6e] = 0x253C;
+    t[0x6f] = 0x23BA;
+    t[0x70] = 0x23BB;
+    t[0x71] = 0x2500;
+    t[0x72] = 0x23BC;
+    t[0x73] = 0x23BD;
+    t[0x74] = 0x251C;
+    t[0x75] = 0x2524;
+    t[0x76] = 0x2534;
+    t[0x77] = 0x252C;
+    t[0x78] = 0x2502;
+    t[0x79] = 0x2264;
+    t[0x7a] = 0x2265;
+    t[0x7b] = 0x03C0;
+    t[0x7c] = 0x2260;
+    t[0x7d] = 0x00A3;
+    t[0x7e] = 0x00B7;
+    t
+};
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn table_lengths() {
+        for set in [Charset::Ascii, Charset::British, Charset::DecSpecial] {
+            assert_eq!(table(set).len(), 256);
+        }
+    }
+
+    #[test]
+    fn table_contents() {
+        assert_eq!(table(Charset::British)[0x23], 0xa3);
+        assert_eq!(table(Charset::Ascii)[0x41], 0x41);
+        assert_eq!(table(Charset::DecSpecial)[0x71], 0x2500);
+    }
+}