textarea: reset viewport when auto-resized wrapped content fully fits

[yogirk]

Summary

• Bug: When a host app auto-resizes textarea height to match the current number of soft-wrapped rows (e.g. after each keypress), a stale viewport.YOffset can hide the first wrapped row — even though the entire buffer now fits within the visible area.
• Fix: In repositionView(), if the total soft-wrapped line count fits within the viewport height, snap the viewport to top before applying cursor-following logic.
• Test: Adds a keypress-driven autoresize regression test that simulates the real failure mode: type a long line one character at a time, auto-resize height after each keypress, and assert the first wrapped row remains visible.

Repro

1. Create a textarea with a constrained width and height of 1
2. Type a long line one keypress at a time
3. After each keypress, set the textarea height to the current soft-wrapped row count
4. Observe the first wrapped row disappears even though the content fully fits

Test plan

• New test TestAutoResizeKeepsFirstSoftWrappedRowVisible passes
• All existing textarea tests pass
• Full module go test ./... passes