fix(frontend): stop fit loop committing transient cell-box mis-measurements (#313)

[harshitsinghbhandari]

What

Fixes the badly-clipped Codex orchestrator terminal reported in #313: the grid renders at roughly half width and a fraction of the pane height, with a ghosted/duplicated composer.

Why

#312 added an onRender convergence loop to XtermTerminal that recovered the under-counted rows from #280. But comparing the screenshots:

• Codex Terminal is sort of clipped. can't write, can't scroll. #280 (pre-fix(frontend): recover clipped Codex terminal via onRender convergence re-fit #312): full width, grid clipped to the top third — pure row under-count.
• Codex orchestrator is very badly cclipped #313 (post-fix(frontend): recover clipped Codex terminal via onRender convergence re-fit #312): ~half width and short, with a duplicated composer — a new failure.

FitAddon.proposeDimensions() derives the grid by dividing the host box by the renderer's measured css.cell box (verified in the installed addon source). During the WebGL atlas warm-up that cell box can emit a one-frame transient — on a HiDPI display (the report is DPR 2) a briefly-doubled box halves the proposed cols/rows. The convergence loop committed that single frame's proposal, resized the grid to half, then detached after a few "stable" frames. With the loop detached and the grid stuck, nothing re-fired the PTY resize that makes zellij repaint, so the half-size grid froze and the stale composer was never cleared.

How

• Two-frame confirmation: a differing proposal must repeat identically across two consecutive renders before it is applied. A one-frame transient now only updates a pending value and is never committed.
• Late settle backstop: added 600ms/1200ms fits alongside the existing 50/250ms. By then the atlas and font metrics are unambiguously warm, so even if the loop detached at a briefly-stable wrong measurement, a late re-measure corrects the grid and fires the resize zellij needs to repaint cleanly. fit() is idempotent, so a correctly-sized terminal never reflows.

Testing

• npm run typecheck — clean
• npm test — 140 passed (15 files)

Backend untouched, so the Go gate was skipped.

Fixes #313

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