Build ANSI-to-spans parser using VTE (litmus-28sq)

Add ANSI output parsing to litmus-capture (or a new litmus-parse crate):

• Use vte or termwiz crate to process raw ANSI byte streams
• Intermediate cell grid to handle cursor movement and overwrites
• Collapse adjacent cells with identical attributes into TermSpans
• Input: raw bytes from fixture command output
• Output: TermOutput struct
• Handle: SGR (colors, bold, italic, dim, underline), newlines, basic cursor movement
• Map SGR color codes to TermColor variants:
  • \e[30-37m / \e[90-97m → Ansi(0-15)
  • \e[38;5;Nm → Ansi/Indexed depending on N
  • \e[38;2;R;G;Bm → Rgb(r,g,b)
  • \e[39m / no color → Default

Depends on: TermOutput types in litmus-model

Plan

Architecture

• New module: crates/litmus-capture/src/ansi_parser.rs
• Add vte crate dependency for ANSI escape sequence parsing
• Cell grid approach: track cursor position, attributes per cell, then collapse

Core types

• CellAttrs: fg/bg TermColor + bold/italic/dim/underline
• Cell: character + CellAttrs
• Grid: rows×cols of Cells, cursor position tracking
• AnsiParser: wraps Grid, implements VTE Perform trait

SGR mapping

• 30-37 → Ansi(0-7) fg, 40-47 → Ansi(0-7) bg
• 90-97 → Ansi(8-15) fg, 100-107 → Ansi(8-15) bg
• 38;5;N → indexed fg, 48;5;N → indexed bg
• 38;2;R;G;B → RGB fg, 48;2;R;G;B → RGB bg
• 39 → Default fg, 49 → Default bg
• 0 → reset, 1 → bold, 2 → dim, 3 → italic, 4 → underline

Public API

pub fn parse_ansi(input: &[u8], cols: u16, rows: u16) -> TermOutput

Tests (TDD)

• Plain text without escapes
• Basic SGR colors (30-37, 40-47)
• Bright colors (90-97, 100-107)
• 256-color mode (38;5;N, 48;5;N)
• 24-bit truecolor (38;2;R;G;B)
• Bold/italic/dim/underline attributes
• Reset (SGR 0) clears all attributes
• Newline handling
• Span collapsing (adjacent same-attr cells merge)
• Cursor movement (basic)
• Real git diff output parsing

Summary of Changes

Added ansi_parser module to litmus-capture with:

• Cell grid architecture: VTE-backed parser with cols×rows grid, cursor tracking, and SGR attribute state
• Full SGR support: standard colors (30-47), bright (90-107), 256-color (38;5;N), truecolor (38;2;R;G;B), bold/italic/dim/underline, individual resets (22/23/24)
• Colon-form subparams: handles 38:5:N and 38:2::R:G:B forms used by modern terminals
• Cursor operations: CUU/CUD/CUF/CUB movement, CUP absolute positioning, EL (erase in line), ED (erase in display)
• Span collapsing: merges adjacent cells with identical attributes, trims trailing default spaces
• parse-ansi CLI subcommand: reads raw ANSI from file/stdin, outputs structured TermOutput JSON
• 37 tests covering all SGR mappings, edge cases (empty input, zero dimensions, column overflow), and real-world output patterns

Key design decisions:

• Characters beyond terminal width are clipped (no line wrapping)
• Trailing default-attribute spaces are trimmed; styled spaces are preserved
• Zero dimensions clamped to 1×1 to prevent underflow