feat(tui): quiet statusbar footer + diff syntax highlighting

[tidux]

Justification

Two related TUI polish fixes:

1. Quiet statusbar footer: The previous statusline behavior was unusably noisy on large diverged feature branches. It didn't tell me at a glance how much of the giant red and green numbers were due to branch history divergence, recent working-tree changes, or untracked files. These changes align nori with most other common development tools.

2. Diff syntax highlighting + relative line numbers: Diff output in the TUI now syntax-highlights code using tree-sitter (matching the file extension) and displays relative line numbers, making diffs significantly easier to read inline.

Implementation

Quiet statusbar footer

• Scope footer git stats to tracked working-tree changes against HEAD
• Show untracked, non-ignored files as a compact red ! instead of counting their lines
• Add regression coverage and update TUI docs for the quieter statusline behavior

Diff syntax highlighting + relative line numbers

• Add render/highlight.rs with tree-sitter-based syntax highlighting keyed on file path extension
• Introduce PathAwareFileChange wrapper so DiffSummary can pass file path context into rendering
• Add DiffLineLayout to centralize line-number width and padding calculations
• Show relative (1-based from hunk start) line numbers in diff output

Validation

• just fmt
• cargo test -p nori-tui
• cargo build --bin nori
• cargo test -p tui-pty-e2e
• just fix -p nori-tui

Screenshots

Statusbar: Before vs After

Before:

After:

[tidux]

The reason the old diff count was so enormous in the ZeroClaw repository in particular is that there was a change from "main" to "master" as the tracking branch months ago, and so no amount of committing new changes back to master would ever make the old behavior drop to 0/0.

[tidux]

Diff output was also broken. I added proper file-relative line numbers and syntax highlighting.

[CSRessel]

Will review this later today, or when you request me for review! Just lmk

The git fix is very welcome. I might keep the old git behavior, but as an alternate footer segment option. This change should be the default one. Diff coloring was something I had in progress work on; I can integrate that during review or after this PR