feat(terminal): port Terminal, read-only stream handler and StringMap

Ports ZIG/src/terminal/Terminal.zig - printing with grapheme clustering
and Unicode widths, wraparound, insert mode, charsets, cursor movement,
scroll regions and left/right margins, erase/insert/delete, tabs, modes,
alternate screens, save/restore cursor, DECALN, full reset, palette,
mouse/focus/paste flags, kitty keyboard, titles/pwd, semantic prompts,
hyperlinks, protected mode, synchronized output and resize.

stream_readonly.rs implements ghostty_vt::stream::Handler, applying all
state-changing actions to a Terminal and ignoring queries; it is the
base for termio's side-effecting handler. StringMap uses the onig crate
for Oniguruma-compatible regexes (needed by Ghostty's URL regex).

Not yet wired: kitty graphics (hooks and dirty flags are in place where
Zig calls into it, storage is being ported separately) and the tmux
control-mode viewer.

Test Plan:
- cargo test -p ghostty-terminal (849 passed, 5 ignored; all 372
  Terminal and 33 stream_readonly tests ported; ignored ones are blocked
  on the PageList parity pass)

crates/ghostty-terminal/src/screen.rs

@@ -361,14 +361,7 @@ pub enum SemanticContentSet {
     Input(InputClear),
 }
 
-/// A string and a map from each byte of the string to the pin it came from.
-///
-/// TODO(integration): replace with `string_map::StringMap` once ported.
-#[derive(Debug, Clone, Default, PartialEq, Eq)]
-pub struct StringMap {
-    pub string: String,
-    pub map: Vec<Pin>,
-}
+pub use crate::string_map::StringMap;
 
 #[derive(Debug, Clone, Copy)]
 pub struct SelectionString {