fix(web): replay cursor movements the way a terminal paints them

Carriage return and backspace only MOVE the cursor; neither erases. Both of
my earlier approximations were wrong, and I checked each case against a real
terminal rather than reasoning about it:

`100%\rOK` shows `OK0%`, not `OK` — the redraw is shorter than the frame
beneath it, so the tail stands. `abc\b` still shows `abc`, not `ab` — a
trailing backspace has nothing to overwrite. `\x1b[31mgone\rkept` paints
`kept` RED, because a carriage return does not reset the graphic state, which
one of my own tests had asserted the opposite of.

Both now replay into a per-line column buffer with SGR state stamped per
column, as a terminal stores it per cell. That gives the partial-overwrite
case its real result too: red `bad`, three backspaces, then `ok` shows `okd`
with the `d` still red, since `ok` reached only two of the three cells.

The presenter description now also renders at every site. An expanded row
draws it itself — the collapsed summary is hidden while open, so otherwise
the description was visible only collapsed, the opposite of "above the card"
— and the details panel draws it above the card as well.

Three of my own tests encoded the wrong semantics and were corrected with
their behavior, and the emit loop's gap-filling arm was removed as
unreachable: `\r` and backspace only move left, so no column can be unwritten.

packages/client/ui-primitives/tests/ansi.spec.ts

@@ -151,8 +151,26 @@ describe('parseAnsiLines: carriage returns', () => {
     expect(onlySpan('10%\r55%\r100%')).toEqual({ text: '100%', style: undefined })
   })
 
-  it('drops the SGR codes that preceded a discarded redraw', () => {
-    expect(onlySpan(`${ESC}[31mgone\rkept`)).toEqual({ text: 'kept', style: undefined })
+  it('leaves the tail of a longer frame standing under a shorter redraw', () => {
+    // Verified against a real terminal: `100%\rOK` paints `OK0%`. A carriage
+    // return only moves the cursor, so the two columns the redraw never reaches
+    // still hold the frame beneath — truncating to the last `\r` would lose them.
+    expect(onlySpan('100%\rOK')).toEqual({ text: 'OK0%', style: undefined })
+    expect(onlySpan('abcdef\rXY')).toEqual({ text: 'XYcdef', style: undefined })
+  })
+
+  it('clamps a backspace run at the line start rather than going negative', () => {
+    // More backspaces than characters: the cursor stops at column 0, so the
+    // following write simply overwrites from there.
+    expect(onlySpan(`ab${BS}${BS}${BS}${BS}xyz`)).toEqual({ text: 'xyz', style: undefined })
+  })
+
+  it('keeps SGR state in force across a redraw, as a terminal does', () => {
+    // Verified against a real terminal: `\x1b[31mgone\rkept` paints `kept` RED.
+    // A carriage return moves the cursor; it does not reset the graphic state,
+    // so the redraw inherits the color the discarded frame was written with.
+    expect(onlySpan(`${ESC}[31mgone\rkept`))
+      .toEqual({ text: 'kept', style: { color: 'var(--dsw-alias-state-error-primary)' } })
   })
 
   it('preserves both lines of a CRLF pair instead of treating it as a redraw', () => {
@@ -184,6 +202,17 @@ describe('parseAnsiLines: backspaces', () => {
     ])
   })
 
+  it('treats a trailing backspace as a cursor move, not a delete', () => {
+    // Verified against a real terminal: `abc\b` still shows `abc`. Only a later
+    // write overwrites; a backspace with nothing after it erases nothing.
+    expect(onlySpan(`abc${BS}`)).toEqual({ text: 'abc', style: undefined })
+    // Same at a line boundary: the newline ends the line before any overwrite.
+    expect(parseAnsiLines(`abc${BS}\ndef`)).toEqual([
+      [{ text: 'abc', style: undefined }],
+      [{ text: 'def', style: undefined }],
+    ])
+  })
+
   it('steps over an SGR sequence instead of erasing its bytes', () => {
     // `abc` reset then two backspaces then `XY`: erasing the reset's bytes would
     // corrupt it and repaint the rest of the line with whatever the remainder
@@ -202,14 +231,21 @@ describe('parseAnsiLines: backspaces', () => {
     ]])
   })
 
-  it('applies the overwrite after a carriage-return redraw, not before', () => {
-    // The redraw wins first; the backspace then erases inside what survived.
-    expect(onlySpan(`old\rnew${BS}`)).toEqual({ text: 'ne', style: undefined })
+  it('replays a redraw and a trailing backspace as pure cursor moves', () => {
+    // Verified against a real terminal: `old\rnew\b` shows `new`. The redraw
+    // repaints all three columns and the trailing backspace only moves the
+    // cursor left — nothing overwrites the `w`, so nothing is lost.
+    expect(onlySpan(`old\rnew${BS}`)).toEqual({ text: 'new', style: undefined })
   })
 
-  it('keeps the run\'s style while erasing its own characters', () => {
-    expect(onlySpan(sgr('31', `bad${BS}${BS}${BS}ok`)))
-      .toEqual({ text: 'ok', style: { color: 'var(--dsw-alias-state-error-primary)' } })
+  it('overwrites only the columns the later write reaches, keeping the rest styled', () => {
+    // Verified against a real terminal: red `bad`, three backspaces, then `ok`
+    // shows `okd` — the cursor returned to column 0 and `ok` overwrote two of
+    // the three columns, so the untouched `d` keeps the run's red.
+    expect(parseAnsiLines(`${sgr('31', 'bad')}${BS}${BS}${BS}ok`)).toEqual([[
+      { text: 'ok', style: undefined },
+      { text: 'd', style: { color: 'var(--dsw-alias-state-error-primary)' } },
+    ]])
   })
 })