[deevus]

Summary

• Add Markdown formatter support for [s]...[/s] as ~~...~~
• Add direct formatter coverage for standalone, mixed inline, and nested strikethrough content
• Update Markdown snapshots and add a portable Zig snapshot checker that supports jj-only and Git workspaces

Fixes #2

Verification

• mise exec -- zig fmt --check build.zig tools/check_snapshots.zig src/formatters/markdown.zig
• mise exec -- zlint --deny-warnings build.zig tools/check_snapshots.zig src/formatters/markdown.zig
• mise exec -- zig test src/root.zig --test-filter strikethrough
• mise exec -- zig test src/root.zig --test-filter "single lines"
• mise exec -- zig test src/root.zig
• mise exec -- zig build
• mise exec -- zig build test --summary all

Also verified dirty snapshot detection with the Zig checker in this jj workspace.