feat: adds a progress/cancel feature to Context for the c2pa-rs sdk

[gpeacock]

Progress/Cancel API

Adds a progress reporting and cancellation API to Context.

Attach a callback when building a context and receive structured notifications at each major phase of signing and reading operations.

Return false from the callback — or call context.cancel() from another thread — to abort with Error::OperationCancelled.
let context = Context::new().with_progress_callback(|phase, step, total| { println!("{phase:?} {step}/{total}"); true // return false to cancel });

The callback receives a ProgressPhase enum value, a 1-based step index, and a total step count (0 = indeterminate).

Phases emitted during a typical sign+verify cycle, in order:

VerifyingIngredient → VerifyingManifest → VerifyingSignature → VerifyingAssetHash → Thumbnail → Hashing → Signing → Embedding → (verify after sign) → VerifyingManifest → VerifyingSignature → VerifyingAssetHash → VerifyingIngredient

And during reading:

Reading → VerifyingManifest → VerifyingSignature → VerifyingAssetHash → VerifyingIngredient

VerifyingIngredient reports accurate step/total counts; VerifyingSignature always reports 1/2 (COSE parse) then 2/2 (OCSP + full verify). VerifyingAssetHash fires immediately before the asset is re-hashed for integrity checking — the most time-consuming part of reading large files.

Includes c_ffi_api support

examples/progress.rs — demonstrates the API with elapsed timing output for both sign and read paths, allows cancel.

This API only applies to the context-based flows (Builder::from_shared_context, Reader::from_shared_context). Older convenience methods are unaffected.

[codspeed-hq]

Merging this PR will not alter performance

✅ 32 untouched benchmarks
⏩ 2 skipped benchmarks¹

---

_{Comparing gpeacock/progress_cancel (897abee) with main (88c1273)}

Footnotes

1. 2 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[codecov]

Codecov Report

❌ Patch coverage is 86.30573% with 43 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.89%. Comparing base (88c1273) to head (897abee).

Files with missing lines	Patch %	Lines
sdk/src/claim.rs	76.19%	10 Missing ⚠️
sdk/src/assertions/data_hash.rs	81.81%	6 Missing ⚠️
sdk/src/crypto/ocsp/fetch.rs	0.00%	6 Missing ⚠️
c2pa_c_ffi/src/c_api.rs	76.19%	5 Missing ⚠️
sdk/src/assertions/bmff_hash.rs	79.16%	5 Missing ⚠️
sdk/src/utils/hash_utils.rs	81.81%	4 Missing ⚠️
sdk/src/assertions/box_hash.rs	95.08%	3 Missing ⚠️
sdk/src/builder.rs	91.66%	2 Missing ⚠️
sdk/src/store.rs	95.83%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1927      +/-   ##
==========================================
+ Coverage   76.87%   76.89%   +0.01%     
==========================================
  Files         175      175              
  Lines       42171    42438     +267     
==========================================
+ Hits        32418    32631     +213     
- Misses       9753     9807      +54     

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[tmathern]

Is sdk/test_input a stray test file?