Implement truthful change classification and diff review

[codeacme17]

Tracks #1.

Summary

Implement truthful Changed File classification and on-demand diff review for the selected Worktree.

Scope

• Parse git status --porcelain=v2 -z --branch --untracked-files=all into Conflicts, Staged Changes, Changes, and Untracked groups.
• Allow the same path to appear in Staged Changes and Changes with independent baselines.
• Produce opaque FileId values bound to one Worktree generation, revision, path, and baseline.
• Implement on-demand Staged (HEAD → Index), Changes (Index → Working Tree), Untracked (empty → file), and truthful conflict views.
• Show renames with old/new paths and keep deletions reviewable.
• Default to side-by-side with a Unified toggle and Previous/Next N of M navigation.
• Detect binary, undecodable, over-2-MiB, or over-20,000-line content before rendering a fake text diff; return metadata and safe open actions.
• Disable external diff/textconv execution and bound process output.

Acceptance criteria

• A staged-and-unstaged path appears twice with the correct distinct content.
• Rename, deletion, untracked, binary, oversized, undecodable, and conflict fixtures all produce truthful results.
• Empty groups are hidden and non-empty groups remain ordered Conflicts, Staged Changes, Changes, Untracked.
• Unusual path bytes cannot address another Worktree or escape the Repository.
• Large/binary fixtures cannot freeze or crash the page.
• Navigation remains scoped to the selected Worktree and current group/filter.

Out of scope

• Hunk/line staging, three-way conflict editing, syntax-highlighting guarantees, or file mutation.

Dependencies

• Discover registered Worktrees with generation-safe identity #6, Implement versioned refresh, operation lanes, and reconciliation #7, and Build the adaptive Repository and Worktree overview #8.

PRD coverage

• FR3; AC-05 and AC-06.

Verification

• Real Git parser/diff integration tests, binary/encoding/size fixtures, path fuzzing, and UI diff navigation tests.