Add classified blob reads

[andrew]

Add InspectBlob and BlobResult so callers can retain bounded blob content alongside a magic.Result. Complete reads use Detect, truncated reads use DetectPrefix, and the existing Blob API keeps its NUL-based behavior. Document the new API and cover text, prefix truncation, binary signatures, invalid UTF-8, UTF-16, NUL handling, empty blobs, and input errors.

[Copilot]

Pull request overview

Adds a new blob-reading API that preserves bounded content and returns structured content classification, while keeping the existing Blob NUL-based binary detection semantics intact.

Changes:

• Introduces InspectBlob and BlobResult for bounded reads with magic.Detect / magic.DetectPrefix classification.
• Refactors Blob to share a common readBlob helper while preserving its original behavior.
• Updates package/docs and expands tests/benchmarks to cover classification, truncation, encodings, NUL handling, and error paths.

Reviewed changes

Copilot reviewed 6 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
test_helpers_test.go	Generalizes git test helpers to testing.TB so they can be reused from benchmarks.
README.md	Documents InspectBlob and clarifies how it differs from the existing Blob behavior.
go.sum	Adds checksums for the new github.com/git-pkgs/magic dependency.
go.mod	Adds github.com/git-pkgs/magic v0.1.0 as a direct dependency.
doc.go	Updates package docstring to reflect blob content classification capability.
blob.go	Adds InspectBlob/BlobResult and refactors shared blob read logic into readBlob.
blob_test.go	Adds benchmarks and expands tests for InspectBlob classification and error reporting.

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