fix: use viewportY offset in Buffer() to read visible viewport

[gluonfield]

Summary

Buffer() reads lines starting from index 0 of the scrollback buffer instead of the visible viewport. This causes Wait+Screen (and SaveOutput) to match against lines that have already scrolled off-screen.

The fix adds viewportY as the starting offset, consistent with how CurrentLine() already works in the same file.

Changes

One-line change in testing.go:

-buf, err := v.Page.Eval("() => Array(term.rows).fill(0).map((e, i) => term.buffer.active.getLine(i).translateToString().trimEnd())")
+buf, err := v.Page.Eval("() => { const s = term.buffer.active.viewportY; return Array(term.rows).fill(0).map((e, i) => term.buffer.active.getLine(s + i).translateToString().trimEnd()); }")

Reproduction

This tape fails on the current main but passes with this fix:

Output "/tmp/test.gif"
Set Height 300
Type "seq 10"
Enter
Sleep 1s
Wait+Screen /9/
Type "echo foo"
Enter
Sleep 1s

Stock VHS reports: timeout waiting for "Screen 9" to match 9; last value was: > seq 10 1 2 3 4 5 — it's reading from the top of the scrollback instead of the visible viewport.

Test plan

• go test ./... passes
• Reproduction tape passes with patched binary
• Verified CurrentLine() already uses viewportY (consistent approach)

Fixes #659