terminal: keep a 133;P primary prompt on its own line and drop wrap padding

[lawrencecchen]

Summary

Follow-up to #245. Cloud terminals (cmux-tui, bash with ble.sh) still left prompt fragments after an aggressive resize.

• 133;P primary prompt. With ble.sh active, Ghostty's bash integration marks the prompt with 133;P;k=i instead of 133;A, because ble.sh cannot track A's fresh-line. ble.sh pads a partial line past the right edge like zsh PROMPT_SP, so the prompt still starts at column 0 of a soft-wrapped row. An explicit primary prompt start (k=i or no kind) now breaks that wrap the same as 133;A. Continuation (k=c, k=s) and right (k=r) prompts keep their wrap.
• Padding. When the wrap is broken, the trailing run of unstyled plain spaces on the row above is cleared. That padding only forced the new row. Kept as text, it reflows into extra blank rows of the partial line whenever the terminal narrows. A shell redraw computed for the old width can then land in those rows and print the prompt into the partial line again. The zsh % mark is styled, so the trim stops there.

Testing

On a Blacksmith testbox (Linux, Zig 0.16.0):

• Terminal: OSC133P after a padded partial line keeps the prompt on its own line across resize: red at f1906ae (expected 1, found 0, the prompt joined the padded row), green at c318e78. The padding assertion (narrowing to 5 columns keeps the cursor on row 1) is new in that test.
• Terminal: OSC133P continuation prompt keeps a soft wrap passes.
• zig build test-lib-vt: one failure, kitty temporary file medium preserves bool ABI, which also fails at the earlier pin on Linux.
• End to end in cmux-tui with the Cloud VM's exact shell setup (/bin/bash, /etc/cmux/bashrc, prompt.bash, ble.sh nightly), a partial line, and 81 width changes at 4 ms intervals through an attached client: main was clean in 0 of 6 runs; this change plus cmux-tui shell injection was clean in 6 of 6.

🤖 Generated with Claude Code

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Summary by cubic

Keeps 133;P primary prompts on their own line in Cloud terminals (cmux-tui, bash with ble.sh) so resize no longer leaves prompt fragments behind. An explicit primary prompt start (k=i) now breaks a soft wrap the same as 133;A, and the wrap-padding spaces are cleared so they don't reflow into extra rows when the terminal narrows.

• Continuation (k=c, k=s) and right (k=r) prompts keep their existing wrap behavior.
• Padding removal only applies to trailing runs of unstyled plain spaces, so styled marks like zsh's % are preserved.
• Adds tests for primary prompt line-breaking across resize and continuation wrap preservation.

^{Written for commit c318e78. Summary will update on new commits.}

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📥 Commits

Reviewing files that changed from the base of the PR and between e30b480 and c318e78.

📒 Files selected for processing (2)

• src/terminal/Screen.zig
• src/terminal/Terminal.zig

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