v0.1.0-alpha.69

A second renderer that draws the grid as a document, opt-in behind
KUROTTY_HTML_RENDERER=1, plus the Dia-inspired window chrome.

The HTML renderer is level with Metal on a full repaint (2.23-2.35ms
against 2.29-2.53ms) and adds nothing to the bundle: WebKit is the
system's. Nothing changes without the flag.

Alongside it, a conformance contract that feeds every TerminalFrame
input to both renderers one field at a time, so a renderer that drops
one is caught by the suite rather than by a screenshot.

Two bugs in existing code found on the way, both fixed: double-click
word selection did nothing once the screen had scrolled into scrollback,
on either renderer, and the HTML path drew the grid at the page origin
while the pointer resolved cells against the frame's padding.