feat(terminal): text selection + copy (custom model)

Drag to select terminal text and copy it with Ctrl-Shift-C. Built as a
custom selection model (xterm.js-style) rather than native selection,
because the virtual scroll only keeps ~60 rows in the DOM and rebuilds
them every frame — native selection would be destroyed and couldn't span
scrolled-out rows.

core.rs:
- selection_text(start, end) extracts a (row,col)→(row,col) range straight
  from history_rows + screen_cache, so copy is correct across the whole
  buffer regardless of what is currently rendered
- TerminalRow::text_in_cols slices a row by grid columns (using the
  segment `cols` widths), trimming trailing whitespace

component.rs:
- Selection { anchor, head } in logical buffer coords; mousedown/move/up
  drag updates it; point_to_cell maps pixels→cell (padding + scrollTop)
- selection_rects computes per-row highlight rects clipped to the rendered
  window; rendered as an overlay inside the translated rows layer so the
  highlight follows scrolling without splitting text segments
- cell_size signal kept in sync with viewport measurement
- Ctrl-Shift-C copies via navigator.clipboard.write_text; Ctrl-C is left
  untouched (still SIGINT); typing clears the selection

style: .terminal-selection overlay, user-select:none on the grid, rows
above the highlight via z-index.

web-sys: add DomRect, MouseEvent, Navigator, Clipboard.

Adds point_to_cell / selection_rects / selection_text unit tests (16
terminal tests total). Marks P0 done in TERMINAL_ROADMAP.md.

Known follow-up: no auto-scroll when dragging past the viewport edge
(scroll first, then select); wide+combining column slicing is approximate.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Cargo.toml

@@ -39,11 +39,13 @@ wasm-bindgen = "0.2"
 wasm-bindgen-futures = "0.4"
 web-sys = { version = "0.3", default-features = false, features = [
     "BinaryType",
+    "Clipboard",
     "ClipboardEvent",
     "CloseEvent",
     "console",
     "DataTransfer",
     "Document",
+    "DomRect",
     "DomRectReadOnly",
     "Element",
     "ErrorEvent",
@@ -53,6 +55,8 @@ web-sys = { version = "0.3", default-features = false, features = [
     "KeyboardEvent",
     "Location",
     "MessageEvent",
+    "MouseEvent",
+    "Navigator",
     "ResizeObserver",
     "ResizeObserverEntry",
     "WebSocket",