Add a synthetic Ideogram C2PA attribution fixture

A throwaway test CA with leaf subject CN=Ideogram, Inc signs a flat PNG
carrying a trainedAlgorithmicMedia claim. The fixture exercises the
issuer-to-platform attribution end to end through the real reader and pins
the second dimension the project separates: attribution names Ideogram
while signer trust stays untrusted. Recipe documented in the fixtures
README; no keys, vendor bytes, or user uploads are committed.

data/fixtures/README.md

@@ -13,3 +13,23 @@ example from [`adobe/trustmark`](https://github.com/adobe/trustmark) at commit
 - SHA-256: `e58c5825ed7e5d9fb04710ea541b61bd55879cad65554c0d46260aa24b3d0755`
 - Expected signal: TrustMark Variant P, schema 1
 - License: MIT, reproduced in `../licenses/adobe-trustmark-MIT.txt`
+
+
+## Ideogram C2PA attribution
+
+`ideogram-c2pa.png` is a SYNTHETIC signed manifest, not a vendor download: a
+64x64 flat PNG carrying a C2PA claim (`trainedAlgorithmicMedia`,
+`claim_generator "Ideogram/3.0"`) signed by a throwaway test CA whose leaf
+certificate subject is `O=Ideogram, Inc, CN=Ideogram, Inc`. It exercises the
+issuer-to-platform ATTRIBUTION path end to end through the real reader; the
+signer is deliberately untrusted (no trust anchors ship), which is also the
+fixture's second assertion: attribution and signer trust are separate
+dimensions.
+
+- Expected signal: C2PA, platform `Ideogram`, `ai_from_metadata` True
+- Expected signer: `untrusted` (test CA; attribution must not imply trust)
+- Recipe (no keys are committed): create a CA and an ES256 leaf with
+  `digitalSignature` key usage and subject `CN=Ideogram, Inc`, then
+  `c2pa.Builder.from_json(...).sign_file(png, out, Signer.from_info(
+  C2paSignerInfo(b"es256", leaf_pem + ca_pem, pkcs8_key, None)))`.
+- No user upload and no Ideogram artifact contributes pixels or bytes.