Resolve exact Git pre/post blobs and diff semantics

[nia-sg-bot]

Why

DiffGraph must analyze the exact Git state the user requested. The current paths do not consistently resolve immutable pre/post blobs across staged, unstaged, commit-range, rename, deletion, and merge-base semantics.

Direction

Git object identity is the source of truth. AI must never choose files, symbols, or graph edges.

Acceptance criteria

• Resolve exact pre/post object IDs, paths, modes, and refs for unstaged, staged, two-dot, and three-dot comparisons.
• Correctly handle added, deleted, renamed, binary, missing, and undecodable files.
• Preserve old/new paths and immutable blob/content identities in artifact provenance.
• Respect pathspec/file scope without silently widening it.
• Return structured warnings rather than converting resolver/parser failures into false changes.
• Add deterministic fixtures covering every comparison mode and edge case.

Enables

Tree-sitter extraction, reproducible schema fixtures, fast caching, extension compatibility, and credible benchmarks.