fix(highlighter): preserve bounded syntax context

[owainlewis]

Summary

• Keep line zero plus eight least-recently-used window-boundary syntax checkpoints so warmed distant Rust and Markdown viewports remain bounded across backward and forward jumps.
• Preserve block-quoted backtick and tilde fence context, including injected Rust, synthetic seeds, contents, visible closers, and container-prefix edits.
• Stop the [performance] Bound Rust syntax seed for deeply nested block comments #124 conservative Rust overflow fallback at truncated-line barriers, Markdown fence boundaries, and quoted-container loss while preserving the exact 64-level nested-comment bound.
• Add the combined design and verification record in docs/issues/130-131-plan.md.

Why

The existing checkpoint insertion discarded every later checkpoint during a backward jump, so alternating between distant locations repeatedly rescanned file-depth prefixes. The Markdown checkpoint scanner also missed fences nested under > containers, causing deep injected Rust context loss. After #124, overflow fallback bypassed truncated-line barriers and could keep visible code styled as a comment.

These fixes stay inside the bounded checkpoint/window design. They retain only deterministic window anchors, carry the minimal quote-depth context needed by the scanner, and split conservative overflow at the same boundaries used by normal context recovery.

Test plan

Verified on exact base b3ef75d0ea74b5a13e022a34699396fb878e03ed and head b0040dbe104d6e0d60af0e97ccbba47c007a4a95:

• cargo test highlighter::tests:: -- --nocapture: 44 passed.
• cargo test performance:: -- --ignored --nocapture --test-threads=1: all 4 passed; deep highlighting completed in 2.381s initially and 22.473ms after edit.
• cargo fmt --check.
• git diff --check and staged diff validation.
• cargo test --all-targets: 308 passed, 4 documented performance tests ignored, and all 5 signal cleanup tests passed.
• cargo clippy --all-targets --all-features -- -D warnings.
• cargo build --all-targets.
• Interactive PTY smoke: opened highlighted Markdown, exited with C-x C-c, and reused the restored shell (CORTEX_SHELL_OK).
• Applied the named improve workflow to clarify checkpoint and overflow-boundary invariants without changing behavior.
• Initial independent review found quoted overflow did not stop at container loss. The implementation and a quoted 65-opener barrier/container-edit regression fixed it. Fresh final independent review approved the unchanged highlighter patch against exact base b3ef75d with no findings.

Risks

The Markdown checkpoint scanner intentionally supports block-quote containers without becoming a complete CommonMark parser. A quoted fence requires its block-quote container on opener, contents, and closer. Losing that container ends the bounded fence context.

Rust block-comment nesting above 64 levels remains conservative by design. Normal Rust resumes at a truncated-line barrier. Fenced Markdown also resumes at its real closer, a truncated-line barrier, or loss of the required quote container.

Checkpoint memory remains capped at line zero plus eight recent anchors. An edit retains checkpoints at or before its first changed line and invalidates later contexts that may be stale.

Related issue

Closes #130
Closes #131
Closes #142

[owainlewis]

@codex review

[owainlewis]

Merged-main verification complete.

Merge commit: 0e9ad9293734a0384fb46e8e839cd4d891b74062 (current origin/main).

Post-merge CI: https://github.com/owainlewis/cortex/actions/runs/30405444201

Rust checks passed in 1m27s, including security workflow policy, formatting, strict Clippy, tests, and release build. Issues #130, #131, and #142 are closed. The original #141 review thread is resolved with commit/PR and focused regression evidence.

[chatgpt-codex-connector]

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