feat(render): draw the cursor in screenshots

[Ayman Bagabas (aymanbagabas)]

Third of a three PR stack: #84 → #85 → #86. Only the last commit is new here; a cross-repo PR cannot be based on a fork branch, so GitHub shows the whole stack. Review feat(render): draw the cursor in screenshots.

A screenshot showed the grid but never showed where the terminal was about to write. You could not tell an editor's caret position, whether a program had hidden the cursor, or which mode it was in.

Before / after

The same session, screenshotted by the same command. The cursor is parked mid-word on The:

before	after

The emulator now reports visibility (DECTCEM, CSI ?25 h and l) and shape (DECSCUSR, CSI Ps SP q). The renderer draws the cursor after the text pass: a block fills the cell, an underline sits on its bottom edge, a bar on its left.

Visibility matters as much as position. Full-screen programs hide the cursor while repainting, so a screenshot that ignored DECTCEM would show one parked wherever the last write happened to land.

The cursor never hides content

A block redraws the character beneath it in the cell background, the way a terminal does. The redraw goes through the same path as the text pass, which keeps right two cases a naive redraw gets wrong:

case	naive redraw	here
double-width (日)	block covers the left half, glyph squashed into one cell	block spans both cells, glyph drawn at full width
nerd font glyph	emitted as text the font has no glyph for, so the block swallows it	redrawn as the same vector glyph

double-width, covered across both cells	nerd font glyph, redrawn as a glyph

The cursor follows a color a program sets

OSC 12 sets the cursor color and OSC 112 resets it, both from #85. The cursor had nothing to paint until this PR, so this is where they become visible. One session, screenshotted after each step, cursor parked on the q:

default	OSC 12 sets #ff8700	OSC 112 resets

The default cursor color follows the profile foreground, so the block is the same grey as the text. The q stays legible in all three.

Notes for review

• Blink is deliberately not represented. A screenshot is a single moment, and a blinking cursor is drawn in the half of the cycle where it is visible.
• Emulator::cursor is relative to the visible screen, so screenshot --full offsets it past the scrollback above.
• Alacritty's HollowBlock (what it draws for an unfocused window) maps to Block, since a headless terminal has no notion of focus. Its Hidden shape means the same as the mode being off.
• 2 conformance cases, which run against every backend, cover hide/show and the three shapes. 6 renderer tests cover each shape's rectangle, the readable character, the double-width span, the vector glyph redraw, a hidden or out-of-range cursor, and the runtime color.

[Ayman Bagabas (aymanbagabas)]

cpendery (@cpendery) third of the stack, on top of #84 → #85. Only the last commit is new here. Please review #84 and #85 first.

[Ayman Bagabas (aymanbagabas)]

Superseded: reopening this from a branch in microsoft/shell-use rather than my fork, so the three can be linked as a real stack. GitHub does not allow a pull request from a fork to join a stack, and the head repository of an existing PR cannot be changed.

Same commits, same branch name; the replacement is linked below.

[Ayman Bagabas (aymanbagabas)]

Reopened as #96, based on #95 so the diff shows only this layer.