Playbook editor: support triple-backtick fenced code

[edwin-zvs]

Summary

• extend the source-preserving single-backtick editor contract to completed exact triple-backtick spans on one source line
• hide completed triple delimiters while highlighting the body in TUI and web; preserve exact Markdown source, Unicode offsets, cursor geometry, and collaboration offsets
• make Backspace at the rendered right boundary remove the complete closing triple delimiter, reveal the opening delimiter/body source, and allow retyping the closer to restore formatting
• keep multiline and incomplete fences literal and line-preserving in both clients, with smart clips, attachments, action links, headings, and list-aware editing suppressed inside them
• preserve current main's Tab/Escape selection-menu focus model and readable inline-code foreground contrast for both single- and triple-backtick rendering
• rename the duplicate inline-code decision to specs/0204-playbook-code-editing.md and evolve it to cover both single- and triple-backtick behavior

Latest-main integration

• merged origin/main at a419c1c via merge commit 9f126ac
• retained Tab-to-enter, focused Tab/Shift-Tab cycling, first-Escape menu dismissal, and post-dismissal selected-list indentation from fix(tui): let Tab focus Playbook selection menu #1285
• routed triple-backtick highlighting through the same readable foreground pair introduced for inline code by fix(playbook): improve inline code contrast #1284

Verification

• cargo build
• cargo test -p construct-cli --bin construct playbook (352 passed; includes single/triple delimiters, Backspace/retype, CJK, painted-buffer differential geometry, and selection-menu tests)
• cargo test -p construct-e2e --test playbook_view (8 passed)
• cargo test -p construct-e2e --test web_smoke (2 passed)
• rustfmt --edition 2021 --check crates/cli/src/playbook_markdown.rs
• git diff --check

Multiline constraint

The Playbook editor keeps a stable one-source-line/one-edit-row and collaboration-offset model. Therefore multiline fences remain visible literal source instead of collapsing opener/closer rows differently across clients. The completed one-line triple form provides delimiter-hidden editing parity without violating those invariants.