[bug] Preserve fenced-code highlighting inside Markdown block quotes

[owainlewis]

Goal

Preserve fenced-code and injected-language context when a Markdown fence is nested in a block quote and its opener is above the viewport.

Evidence

At exact main 1d6d60189e4b350649f591a858746368de78064c, markdown_fence and markdown_fence_closes in src/highlighter.rs only trim up to three spaces and require the fence marker to be the first remaining byte. A valid line such as > ```rust is not recognized, so deep bounded viewports are seeded as normal Markdown and lose injected Rust highlighting.

Original review thread: #119 (comment)

Acceptance criteria

• Markdown checkpoint scanning recognizes opening and closing fenced blocks inside block quotes.
• Container prefixes are handled consistently on the opener, contents, and closer.
• Deep viewports preserve injected Rust highlighting when the quoted opener is above the window.
• Ordinary unquoted fences and bounded-work guarantees remain unchanged.

Checks

• Add focused shallow and deep regressions for quoted backtick and tilde fences, including injected Rust.
• Cover closing the quoted fence inside the visible window and editing a container prefix.
• Run focused highlighter tests and opt-in highlighting performance checks.
• Run cargo fmt --check, cargo test --all-targets, strict Clippy, and an all-target build.

Scope

Support block-quote container prefixes without implementing a complete Markdown parser in the checkpoint scanner.

[owainlewis]

Implemented in ready PR #144: #144

Commit: b0040db on exact base b3ef75d.

Verification: shallow and deep quoted backtick/tilde fences, injected Rust, visible closers, and container edits are covered among 44 passing focused highlighter tests. All 4 opt-in performance checks, 308 all-target tests, 5 signal cleanup tests, formatting, strict Clippy, and build passed. Fresh independent exact-base review approved with no findings. Protected CI is pending.