RT-298: scroll through a diff line taller than the pane row by row

[m4ttheweric]

RT-298. glitter's diff pane soft-wraps, but a changed line that wrapped to more rows than the pane (minified JS, one-line JSON) pinned its first row on top. Its middle and end could not be reached.

What changed

• moveDiffCursor takes one step at a time: on a cursor line taller than the pane, a step scrolls it one row until its far end is on screen, then the cursor moves to the next line
• a tall line entered from below opens at its bottom, so reading upward never jumps
• the wheel goes through the same steps, so a tick spends its rows inside the line first and carries the rest onto the next lines
• the offset into a tall cursor line (diffRowOff) belongs to the line it was set on: a stage keeps the rows being read, a resize keeps the place within the line, and a different line landing under the cursor (a discard, the same path in another commit) opens at its first row
• tallCursorSpan reads the shared diffRowIndex and the last pane height, so the index stays the one row map for render, viewport and diffHit
• staging, clicks and space/s/d still act on the whole line

Verification

• Go tests: stepping down row by row then on to the next line, entering from below at the bottom, a wheel tick spilling past the line's end, a render keeping a mid-line top; the old first-row-pinned test rewritten for the new behaviour. go test ./... and go vet ./... clean
• review (Opus, standing in for rate-limited CodeRabbit): an absolute diffTop carried a mid-line position onto a different tall line and lost the place on resize; fixed and pinned by tests that fail on the absolute version
• live in glitter (130x30, widened to 160 mid-line keeping the 5-row offset, a 90-function one-line app.min.js): down steps one row at a time through the line and then onto const b/tail; up from below opens the line at END-OF-LONG-LINE; two wheel ticks move 6 rows; toggling the stage mid-line keeps the view

🤖 Generated with Claude Code

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