fix(layout): honor DECCOLM logical widths

[forketyfork]

Solution

vttest enables DEC mode 40, switches to 80-column DECCOLM, and then draws part of the cursor movement border by relying on cursor-forward clamping at the right edge. Architect delegated that sequence to ghostty-vt, but the next layout pass treated the 80-column model as stale and resized it back to the current tile or window width. That made the lower plus border start too far to the right.

This keeps ghostty-vt's 80/132-column logical width while Architect's own layout target is unchanged. Window, font, grid, and full/grid changes still reset the terminal to the computed size, so normal reflow behavior remains intact. The resize policy now has regression coverage for preserved DECCOLM widths, reset on a real layout change, and correction of ordinary terminal-width drift.

Test plan

• Run vttest "Test of cursor movements" in Architect and confirm the lower + border aligns with the upper one in the 80-column pass.
• Resize the Architect window after the test and confirm the terminal reflows to the current window/grid size.

[Copilot]

Pull request overview

This PR fixes a layout/terminal-model interaction where applications using DECCOLM (80/132-column mode) could have their ghostty-vt logical width incorrectly “corrected” back to the current tile/window target width on the next layout pass, causing cursor-movement drawing (e.g., vttest borders) to misalign.

Changes:

• Preserve ghostty-vt’s 80/132-column logical width when DECCOLM mode is active and Architect’s computed winsize target is unchanged.
• Add regression tests covering (1) preserving DECCOLM width, (2) resetting to layout width on real layout changes, and (3) correcting ordinary non-DECCOLM width drift.
• Document DECCOLM support in the README and describe the resize policy behavior in the architecture docs.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/app/layout.zig	Avoids resizing the terminal model when DECCOLM is active and the PTY/layout target hasn’t changed; adds focused regression tests for the new policy.
README.md	Notes VT-compatible DECCOLM (80/132-column) mode handling as a supported feature.
docs/ARCHITECTURE.md	Documents the updated terminal resize behavior, including DECCOLM logical-width preservation and reset conditions.

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