Playbook editor: render fenced code as coherent blocks

[edwin-zvs]

Summary

• render completed multiline fences as one continuous full-width code surface in the TUI, including wrapped rows and hidden delimiter rows
• join web fence rows into one seamless block with rounded outer corners only
• preserve exact Markdown source, hidden delimiter offsets, CJK/collaboration geometry, incomplete-fence inertness, Backspace/retype behavior, and the PR fix(tui): honor dismissed Playbook selection menu #1286 selection-menu dismissal guard
• update accepted spec 0204 and Playbook docs with the block-level UX decision

Rendering model

The TUI paints the block background directly into otherwise-empty frame cells before rendering text. It adds no glyphs or padding, so wrapping, cursor positions, selections, remote offsets, and hit testing stay on the source-stable model from PR #1287. The web keeps one DOM row per source line and uses contiguous full-content-width backgrounds with top corners only on the opener and bottom corners only on the closer.

Regression coverage

• Ratatui frame test proves opener, wrapped CJK body, and hidden closer fill one continuous full-width block while adjacent Markdown stays outside it
• Chromium contenteditable test proves full editor-width geometry, zero seams, outer-corner grouping, exact offsets, inert extensions, and Backspace/retype restoration
• the Playbook browser artifact now includes the fenced block treatment

Verification

• cargo build
• cargo test -p construct-cli --bin construct playbook -- --nocapture (356 passed)
• cargo test -p construct-e2e --test playbook_view -- --nocapture (8 passed)
• cargo test -p construct-e2e --test playbook_view web_playbook_view_full_parity -- --nocapture (focused final rerun passed)
• cargo test -p construct-e2e --test web_smoke -- --nocapture --test-threads=1 (2 passed)
• git diff --check

Binary

This change ships in construct. Worktree debug binary: /Users/moon/agentd/.claude/worktrees/playbook-fenced-code-block/target/debug/construct

[edwin-zvs]

Browser parity artifact showing the coherent multiline fenced-code block: