## Summary

`muse blame <file> --range` attributes lines to a commit whose own
`files_modified` list does not include that file, and whose blobs for the
file are byte-identical to its parent — i.e. the blamed commit provably
never touched the file.

## Repro

```bash
cd ~/ecosystem/musehub
muse blame src/scss/_layout.scss --range 38-41 --json
```

Returns attribution to `sha256:659f09b2a6570076b0ec01897c95ce391467b1a11fb8fa6d3c12e88b2e0e1c77`
("fix: update all list_domains/list_repos callers to cursor pagination API").

```bash
muse read sha256:659f09b2a6570076b0ec01897c95ce391467b1a11fb8fa6d3c12e88b2e0e1c77 --json \
  | jq '.files_modified'
```

```json
[
  "musehub/api/routes/musehub/ui_domains.py",
  "musehub/mcp/dispatcher.py",
  "musehub/mcp/resources.py",
  "musehub/services/musehub_mcp_executor.py"
]
```

`src/scss/_layout.scss` is not in that list. Confirming the file content is
identical between the blamed commit and its parent:

```bash
diff <(muse cat src/scss/_layout.scss --at sha256:659f09b2a6570076b0ec01897c95ce391467b1a11fb8fa6d3c12e88b2e0e1c77) \
     <(muse cat src/scss/_layout.scss --at sha256:9a6e44e84b66174d5e1a275f1d011bb9b100a3bbafc102f7620b36e8db53d1df)
# zero output — byte-identical
```

## Expected

`muse blame` should attribute each line to the nearest ancestor commit that
actually changed it (per its manifest / diff), never a commit whose
`files_modified` excludes the file entirely.

## Impact

Blame output cannot be trusted for archaeology (e.g. "why does this rule
exist, when was it introduced") — it silently returns plausible-looking but
false commit history, which is worse than an error since it's not obviously
wrong without independent verification.