chore: bump c2pa-rs to v0.91.0 (#325)

* Fix test_sdk_version regression: gate on an env var, not file presence

#327's parse_native_version() fix preferred c2pa-rs-preflight-ref.txt
whenever it existed in the checked-out tree. That broke the *real*
build.yml jobs on #325: that PR adds the ref file to the branch, so an
ordinary tests-unix/tests-windows run (which downloads and installs
the actual pinned release, unrelated to the preflight workflow) picked
up the file too and wrongly expected the RC's version string, failing
with e.g. "'0.91.0-rc.3' not found in '0.90.22'".

The file's mere presence was never a reliable signal -- only
test-c2pa-rs-source-build.yml's own "Run tests" step actually builds
from that ref. Gate on a new C2PA_PREFLIGHT_RUN env var that only that
step sets instead.

Verified locally: with the pinned 0.90.22 installed and no env var
set, all 444 tests pass regardless of whether
c2pa-rs-preflight-ref.txt happens to exist in the tree.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

* Point the RC preflight at c2pa-rs 0.91.0-rc.3

This is the entire diff this PR now carries on top of #328: pin the
preflight workflow at c2pa-rc-v0.91.0-rc.3 so it builds from that git
tag (no crates.io publish, no prebuilt GitHub release binaries for an
RC) and runs the full unit test suite against it.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

* Point the RC preflight at c2pa-rs 0.91.0-rc.3

This is the entire diff this PR now carries on top of #328: pin the
preflight workflow at c2pa-rc-v0.91.0-rc.3 so it builds from that git
tag (no crates.io publish, no prebuilt GitHub release binaries for an
RC) and runs the full unit test suite against it.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

* Preflight against c2pa-rs 0.91.0-rc.4

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

* chore: bump c2pa-rs to v0.91.0

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>
Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: 3 people

c2pa-native-version.txt

@@ -1 +1 @@
-c2pa-v0.90.22
+c2pa-v0.91.0