Add ASCII rendering fallback for terminals without image protocols

[jpeletier]

Summary

Adds a Unicode box-drawing fallback (--protocol ascii) so serie works in any terminal — gnome-terminal, alacritty, xterm, Windows Terminal, etc. — without needing the Kitty or iTerm2 image protocols. Auto-detect now uses it as the default for terminals it cannot identify, instead of unconditionally trying iTerm2.

What changed

• New ImageProtocol::Ascii variant, selectable via --protocol ascii or core.option.protocol = \"ascii\" in config.
• Auto-detect rewritten — picks Kitty for KITTY_WINDOW_ID / Ghostty, iTerm2 when TERM_PROGRAM matches iTerm.app / WezTerm / mintty / vscode (or LC_TERMINAL=iTerm2), and ASCII otherwise.
• Single-width ASCII renders one character per branch column with ● ─ │ ╭ ╮ ╰ ╯ (rounded) or ┌ ┐ └ ┘ corners with --graph-style angular.
• Double-width ASCII renders two characters per column, with ○ for merge commits and > / < arrows in the filler slot showing which side the merged-in branch enters from. Merges are detected by LeftTop / RightTop corner edges in the row (branch-source commits stay as ●).
• 38 new unit tests cover edge_directions, ascii_symbol for both styles, render_ascii_row single + double width, merge-vs-branch-source discrimination, and arrow placement. Test count: 83 unit + 19 integration (was 45 + 19).
• Docs updated: README, FAQ (previously claimed there was no fallback), introduction, command-line options, config file format, compatibility page, and config.schema.json.

Example

A synthetic repo with three feature branches merged back to master, rendered with --protocol ascii --graph-width double --graph-style rounded:

○<╮     ce2322a merge feature-c
│ ●     8d7fabb c1
●─╯     9c67479 m5
○<╮     9be857e merge feature-b
○<┼─╮   00516ad merge feature-a
│ ● │   02109cc b1
│ │ ●   84f0caf a2
●─╯ │   75afbd5 m4
│   ●   c5ec6e2 a1
●───╯   ca95409 m3
●       d676cb3 m2
●       468b4b8 m1
●       1efc5bb init

Each merge commit shows as ○ with a < (or >) arrow pointing into it from the side the merged-in branch curls down to. Branch-source commits (like m5, m3) stay as ● because their corner edges are LeftBottom/RightBottom, not LeftTop/RightTop. Branch colors come from the existing palette.

In single-width mode the same graph collapses to one character per column:

○╮    merge feature-c
│●    c1
●╯    m5
○╮    merge feature-b
○┼╮   merge feature-a
│●│   b1
││●   a2
●╯│   m4
│ ●   a1
●─╯   m3

Test plan

• cargo test --release passes (102 tests total)
• cargo build --release produces a clean binary
• Manual smoke test in gnome-terminal — ASCII fallback renders correctly
• Manual smoke test with --protocol ascii --graph-style angular — produces angular corners
• Manual smoke test with --protocol ascii --graph-width double — produces ○ + arrows on merges

[lusingander]

#66 (comment)

[jpeletier]

Got it--I did this because I was frustrated I downloaded serie and no graph appeared and I had all this black area on the left, I didn't know what was happening so I added this fallback mode. I use Ubuntu, but the graph didn't appear in neither the stock console app, Alacritty or with Zellij I use. So here is my fork https://github.com/jpeletier/serie , in case anyone finds it useful.