playbook: place a collaborator's name flag where it can't cover text (#1093)

[edwin-zvs]

Fixes #1093.

A collaborator's name flag was painted unconditionally one row above their caret, straight over whatever was already rendered there. A peer editing one line below turned your own text into gibberish, with no way to tell the badge from the document:

before:   • build tWeb 1ing ZZZ
after:    • build the thing ZZZWeb 1

Both captured from a live TUI with two clients on the same Playbook.

Placement

The badge now takes the first run of blank cells that fits, scanning right from the preferred spot — the row above beside the caret, then the caret's own row, whose trailing blanks are free on any line shorter than the pane.

"Blank" means the cell holds no visible glyph. A cell that is merely styled — a shimmer band, a selection tint — still counts as free: painting a badge over a background is fine, painting it over a character is not.

When neither row has room the badge is dropped. The caret cell itself is already painted in the collaborator's colour, so they stay identifiable — which is the fallback #1093 asked for.

No separator space was added between the text and the badge, and it doesn't need one: the badge carries its own background colour, so ZZZWeb 1 renders as a distinct chip rather than running into the text. Confirmed in the rendered escape sequence (48;2;57;255;136).

Tests

Three, over a hand-built buffer, so the property is pinned directly rather than through a full render:

• playbook_collab_label_never_covers_a_glyph — the occupied case slides right to the trailing blanks; a full row yields no slot at all.
• playbook_collab_label_keeps_the_preferred_slot_when_free — badges already sitting in blank space don't move, so this doesn't relocate flags that were fine.
• playbook_collab_label_may_sit_on_a_styled_blank — a shimmer or selection tint doesn't block placement.

The first failed on its first run and it was the fixture, not the code: a 5-wide badge in a 21-wide buffer where the blank run began at 17 genuinely has nowhere to go. Widened the fixture rather than loosening the search, which would have quietly defeated the point of the test.

Binaries

Only crates/cli changes → construct:

/Users/moon/agentd/.claude/worktrees/fix-playbook-collab-label/target/debug/construct

To see it: open the same Playbook in the TUI and the web UI, and put the web caret on a line whose predecessor has text.