feat(terminal): port Screen, ScreenSet, Tabstops and Selection

Ports ZIG/src/terminal/{Screen,ScreenSet,Tabstops,Selection}.zig on top
of the page/PageList layer. The cursor holds a TrackedPin; Zig's cached
page_row/page_cell pointers become accessor methods. Selection methods
take the PageList explicitly because tracked bounds are handles.

Screen tests use minimal plain-text dump/selection-string helpers that
follow the formatter's semantics; they will delegate to the real
formatter once formatter.zig is ported.

Gaps found in the PageList layer are recorded for the parity pass in
docs/rewrite/notes/ghostty-terminal-pagelist-requests.md; three resize
tests are #[ignore]d pending those fixes (PageList::resize keeps
trailing blank rows Zig drops). Kitty image storage and StringMap are
placeholders.

Test Plan:
- cargo test -p ghostty-terminal (443 passed, 3 ignored; Screen 183/184,
  Selection 18/18, Tabstops 4/5 - skipped tests use allocator failure
  injection)

docs/rewrite/notes/ghostty-terminal-pagelist-requests.md

@@ -0,0 +1,11 @@
+# Requests for page.rs / page_list.rs (from the Screen port)
+
+1. **Pin-based iterators** (Zig `PageList.zig` ~5100-5170): `Pin::{row_iterator, cell_iterator, page_iterator, prompt_iterator}(direction, limit: Option<Pin>)`.
+   The limit pin is inclusive and applies at row granularity. Screen uses a local copy in `pin_iter.rs`.
+2. **`PageList::set_tracked_pin` validity**: it rejects pins with `y >= node.size.rows`. Zig writes the pin raw; `Screen::split_for_capacity` temporarily restores the cursor to the pre-split node, which may have shrunk. Worked around with a clamped stand-in.
+3. **Cross-page row clone**: Zig `Page.cloneRowFrom(other_page, ...)` between two nodes of one list needs two page borrows. Screen implements it with `export_cell`/`import_cell` (`Screen::clone_row_between`). A `PageList::clone_row_between(src_pin, dst_pin)` would be better.
+4. **String allocator probing**: `Page.string_alloc` is private; Zig `cursorSetHyperlink` probes it before growing. Screen instead re-adds the cursor hyperlink after `increase_capacity`. Also `insert_hyperlink` allocates strings before checking set dedup, so near-full pages can fail needlessly.
+5. **`fixup_viewport` must handle `Viewport::Top`** after `erase_rows` (Zig PageList.zig ~3043-3070). Worked around in `Screen::erase_rows`.
+6. **Resize reflow / no-reflow fewer rows** must drop trailing blank rows, including rows holding only bg-color cells (Zig `resizeCols`/`resizeWithoutReflow`). Three Screen tests are `#[ignore = "pagelist: ..."]`:
+   `screen_resize_no_reflow_less_rows_trims_blank_lines`, `screen_resize_less_cols_with_reflow_but_row_space`, `screen_resize_less_cols_to_wrap_wide_char`.
+7. `PageList::reset` and `erase_rows` return `Result`; `erase_rows` with an empty history start point returns `InvalidCoordinate` (Zig would assert). Screen guards/ignores.

docs/rewrite/notes/ghostty-terminal-screen.md

@@ -0,0 +1,34 @@
+# ghostty-terminal: Screen, ScreenSet, Tabstops, Selection
+
+## Ported
+| Zig | Rust | tests |
+|---|---|---|
+| Screen.zig | `screen.rs` (+ `cursor.rs` for cursor.zig `Style`, `pin_iter.rs` helper) | 183 of 184 (`screen/tests/p1..p5.rs`) |
+| ScreenSet.zig | `screen_set.rs` | 1/1 |
+| Tabstops.zig | `tabstops.rs` | 4/5 |
+| Selection.zig | `selection.rs` | 18/18 |
+
+Skipped: `Tabstops: resize alloc failure preserves state` and `selectionString map allocation failure cleanup` (Zig tripwire failure injection).
+Ignored (pagelist limitations, see `ghostty-terminal-pagelist-requests.md`): 3 resize tests.
+Deps added: ghostty-unicode (width in `test_write_string`), ghostty-datastruct (unused so far), log.
+
+## Deviations
+- Cursor `page_pin` is a `TrackedPin`; `page_row`/`page_cell` are accessors: `cursor_pin()`, `cursor_row()/_mut()`, `cursor_cell()/_mut()`, `cursor_page()/_mut()`, `cursor_row_cells()`, `cursor_cell_left/right[_mut](n)`.
+- Errors are `page::Error` (re-exported `screen::Error`). `Screen::reset()` returns `Result`.
+- `Screen::new(Options)`; `Options::new(cols, rows, max_scrollback)` (0 = no scrollback).
+- `clone` is `clone_screen(top, bot)`. `cursor_copy(&Cursor, hyperlink: bool)` copies the hyperlink from the other cursor's heap copy.
+- `append_grapheme(x, cp)` takes a column of the cursor row. `clear_cells(node, y, x0, x1)` takes a half-open column range.
+- Selection methods take `&PageList` (`&screen.pages`) since tracked bounds are handles: `sel.start(&pages)`, `top_left`, `order`, `contains`, `contained_row`, `ordered`, `eql(&pages, &other)`, `adjust(&mut pages, adj)`, `track(&mut pages)`, `deinit(&mut pages)`. `Selection` is `Copy`.
+- `kitty_images` is a placeholder `KittyImages { dirty }` (TODO(integration) until kitty graphics storage lands). `StringMap` is a local struct (TODO(integration)).
+- Minimal plain-text formatter (`plain_format` in screen.rs) backs `selection_string[_map]`, `dump_string`, `dump_string_alloc[_unwrapped]`; TODO(integration): delegate to formatter.rs.
+- Tabstops::unset XORs like Zig (unsetting an unset stop sets it).
+- `cursor_set_hyperlink` does not probe the string allocator (private); it re-adds the link after growth.
+- `Screen::erase_rows` carries a `Viewport::Top` workaround (request 5).
+
+## API overview for Terminal
+`Screen` fields: `pages, no_scrollback, cursor, saved_cursor, selection, charset, protected_mode, kitty_keyboard, kitty_images, semantic_prompt, dirty`.
+Cursor motion: `cursor_up/down/left/right(n)`, `cursor_absolute(x,y)`, `cursor_horizontal_absolute(x)`, `cursor_down_scroll()`, `cursor_scroll_above()`, `cursor_reset_wrap()`, `cursor_mark_dirty()`, `cursor_reload()`.
+Editing: `clear_cells`, `clear_unprotected_cells`, `clear_rows`, `erase_rows`, `split_cell_boundary(x)`, `blank_cell()`, `resize(Resize)`, `scroll(Scroll)`, `scroll_clear()`.
+Style/links: `set_attribute(sgr::Attribute)`, `manual_style_update()`, `start_hyperlink(uri, id)`, `end_hyperlink()`, `cursor_set_hyperlink()`, `cursor_set_semantic_content(..)`.
+Selection: `select(Option<Selection>)`, `clear_selection()`, `select_line/all/word/word_between/output`, `line_iterator`, `prompt_click_move`, `selection_string`.
+Test helper: `test_write_string`.