fix(scan): preserve snapshot digest for diff scans

[charle-z]

Fixes #299.

Preserve the canonical snapshot digest for diff scans so generated git_diff manifests satisfy the scan contract for working-tree, single-commit, and revision-range targets.

Tests:

• bun test tests-ts/compact-diff-scan.test.ts

[charle-z]

After a broader adversarial review, I no longer recommend merging this implementation.

This patch derives committed-diff identity only from kind, baseRevision, and headRevision. That fixes the immediate missing-field failure, but it does not bind the digest to the rendered diff bytes or cover mutable Git replacement/config state, completion-time drift, SHA-256 repository empty trees, or bounded-memory hashing of large binary diffs.

#241 addresses the same failure with the stronger content-bound design and the related revalidation cases. Please treat #373 as superseded by #241.

[charle-z]

After comparing this patch against #241 and its review coverage, I’m withdrawing this PR in favor of #241. This change fixes the immediate missing snapshotDigest, but deriving it only from kind/base/head does not bind the actual rendered diff and leaves mutable Git state, SHA-256 empty-tree handling, completion-time revalidation, and large-diff streaming uncovered. #241 is the more complete direction.