Playbook editor: format multiline fenced code

[edwin-zvs]

Summary

• format completed multiline backtick fences in Playbook TUI and web while preserving exact Markdown source
• retain one editor line per source line, hide only delimiter glyphs, and highlight inert fence content
• make Backspace at the hidden closing boundary remove the full closing run; retyping restores formatting
• preserve Unicode collaboration offsets, CJK cursor geometry, inverse hit testing, and inert smart clips/attachments/action links
• supersede the literal-multiline constraint in spec 0204 and update Playbook docs

Row-stable editing model

Opening and closing delimiter lines keep their source rows. Indentation, info strings, and trailing spaces remain visible; delimiter-only lines render as highlighted blank rows. Source offsets still include every hidden backtick and newline.

The unavoidable UX tradeoff is that distinct source positions within a zero-width hidden delimiter share the same visual caret boundary. Their persisted and collaboration offsets remain distinct and exact.

Incomplete fences remain literal and inert. Backspace at a completed closing boundary reveals that literal incomplete form; retyping the matching closer restores the formatted block without rewriting surrounding source.

Latest-main integration

• based on origin/main at 003fa4b after PR fix(tui): honor dismissed Playbook selection menu #1286
• preserved the selection-menu renderer guard: selection_menu == None returns without painting or retaining hit targets
• the full Playbook TUI suite includes and passes the live selection-menu dismissal/no-repaint regression

Verification

• cargo build
• cargo test -p construct-cli --bin construct playbook -- --nocapture (355 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 rerun passed)
• cargo test -p construct-e2e --test web_smoke -- --nocapture --test-threads=1 (2 passed)
• rustfmt --edition 2021 --check crates/cli/src/playbook_markdown.rs
• git diff --check

Binary

Relevant binary: /Users/moon/agentd/.claude/worktrees/playbook-multiline-fences/target/debug/construct

This PR changes crates/cli and embedded web assets, both shipped through construct.