feat: add -f/--follow to render appended content like tail -f

[DanCard]

What

glow -f file.md renders the file, then appends newly written content to stdout as the file grows — tail -f for markdown. Output is append-only (never repaints), so scrollback survives and piping works.

How

• Watching: fsnotify on the file's parent directory, so atomic-rename saves from editors don't kill the watch; write bursts are debounced (100ms). Only appended bytes are read, tracked by offset.
• Block-safe chunking: markdown can't render mid-block, so pending input is flushed at the last blank line outside a fenced code block, with CommonMark-aware fence tracking (backtick/tilde, fence length, ≤3-space indent, backtick info-string rule).
• Idle flush: a trailing block with no terminating blank line renders after 8s of quiet. If that splits an open code fence, the opening fence line is re-emitted with the next chunk so the remainder still renders as code.
• Truncation/rewrite: like tail -f, a shrunken or replaced file gets a rendered --- divider and a full re-render below prior output.
• Non-markdown files: rendered per complete line, wrapped as code blocks.

--follow requires exactly one local file; stdin, URLs, directories, and combining with --pager/--tui produce clear errors. fsnotify was already a dependency.

Testing

• Unit tests for boundary detection (fences, short closing fences, tilde fences, indented pseudo-fences) and flush behavior (partial block held, open fence held, fence reopen after idle flush, plain-text line flushing).
• Verified end-to-end by scripting appends against the built binary: partial paragraphs held until terminated, fences held across internal blank lines, rewrite produced divider + fresh render, trailing paragraph appeared only after the idle flush.
• go build, go vet, go test ./..., and golangci-lint run clean for the new code.

Known limitations

• Append semantics: mid-file edits fall back to divider + full re-render.
• Reference-style links and setext headings don't resolve across chunk boundaries.

🤖 Generated with Claude Code

https://claude.ai/code/session_0115ibTkoRG9o6Brf1reA1Jj