fix(infinity-agent-cli): don't eat scrollback when a resize races the re-anchor cursor query

[shadaj]

When the terminal is resized continuously (e.g. dragging a pane divider in
Zed), the terminal keeps reflowing while the viewport's re-anchor cursor
query (CSI 6n) is in flight. Acting on the stale reply re-saved the anchor
too high (a growing reflow pulls scrollback rows down, moving the true anchor
with them) and the subsequent clear-from-anchor-down erased the tail of the
assistant output.

• InlineViewport::re_anchor now re-checks the terminal size after every
  cursor-query round-trip and, if the geometry moved again, absorbs the new
  resize and re-queries until stable
• InlineViewport::draw runs re_anchor before computing any
  geometry-dependent values (frame area, ideal viewport position), so a draw
  that absorbs a raced resize is fully consistent with the newest geometry
• Regression test tui_resize_race.rs reproduces the race with a TermOut
  interposer that resizes the emulator right after answering the cursor
  query (before the fix, 6 of 30 output lines were erased)
• Test harness: TuiHarness::spawn_with_term allows spawning the TUI
  against a caller-provided TermOut and event channel

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