core/diff: per-kind line-number fg + side-targeted line color API

[kitlangton]

Summary

Two small additions to the diff renderer, both motivated by real consumer pain:

1. Per-kind line-number foreground colors

Today every line-number digit in the gutter is painted with one lineNumberFg (default #888), even on + / − rows where the sign glyph is already tinted green/red. This adds three optional overrides:

• addedLineNumberFg
• removedLineNumberFg
• contextLineNumberFg

Each defaults to undefined (existing lineNumberFg remains the umbrella default), so behavior is unchanged unless a consumer opts in.

Underneath, LineColorConfig (the per-line override shape used by LineNumberRenderable) gains an optional fg?: string | RGBA so manual setLineColor() callers can also set the digit color, not just background.

before:                              after (with the new options set):
  41                                   41                       (gray)
  43 −  return greeting + name         43 −  return greeting…   (red)
  43 +  return `Hi, ${name}!`          43 +  return `Hi…`       (green)
  ↑                                    ↑
  digit always gray                    digit picks up kind color

2. Side-targeted line color API

The diff-level methods that paint individual lines now accept an optional side parameter:

type DiffSide = "left" | "right" | "both"

diff.setLineColor(line, color, side?)
diff.clearLineColor(line, side?)
diff.setLineColors(map, side?)
diff.clearAllLineColors(side?)
diff.highlightLines(start, end, color, side?)
diff.clearHighlightLines(start, end, side?)

Default is "both", so existing callers are byte-identical. New capability is "left" / "right" for split-view consumers — e.g. ghui, which currently has to do diff as unknown as DiffRenderableRuntimeSides and reach into the private leftSide / rightSide fields to highlight a single side for a PR comment anchor. With this PR, that hack collapses to a single public call.

Test plan

• CI typecheck (local tsc --noEmit is clean for the modified files; pre-existing errors in unrelated tests/dev files remain unchanged)
• bun run test src/renderables/Diff.test.ts src/renderables/__tests__/LineNumberRenderable.test.ts — could not run locally; my checkout's prebuilt @opentui/core-darwin-arm64 is missing the setClearOnShutdown symbol added in recent main, and a from-source rebuild fails with libc symbol errors (Zig 0.15.2 toolchain mismatch). Both predate this PR; CI / a fresh bun install should be fine.
• Manual visual check via packages/examples/src/diff-demo.ts with the new options set, confirming digits in the + / − rows pick up their tinted colors and that setLineColor(..., "left") only paints the left side in split view.

Marking as draft pending those last two checks.