diff: the docked pane starts at its header, under the engine's own head row - #99206

Summary

TLDR: docked, /diff shows one blank row above its header where it showed two.

Docked, /diff no longer pads a blank row of its own above its header: the engine now keeps a docked pane's first row for its close mark, and that row is the blank one.

Notes

Test Plan

• tests/views/pane-view.test.ts: two new cases (the window takes every row under the list; the list takes the wheel from its first row and not from the margin above it), two rewritten (no top pad; the empty state on the built-in's row).
• With the tests alone applied those four fail. With the whole change, claude plugin test mods/diff: 212 pass, 0 fail.
• The mods' own typecheck (tsc -p mods/tsconfig.json): clean.
• Driven in a 40-row terminal at 120 and 160 columns, two changed files: one blank row over the header where there were two, 29 body rows where there were 28; a click on a file row, on [ ask ], on the source picker and on the close mark each lands on its row; a wheel tick moves the list on the list's rows and the body on every other.
• Not docked (the classic renderer, or a terminal under 110 columns): the same screen as before.

Changelog

Fixed /diff showing two blank rows above its header when docked