ENH: Add IsotropicWavelets orphan checkershadow JPEG fixtures (2 CIDs)

Publishes the two orphan `checkershadow_*.jpg` test fixtures used by the
IsotropicWavelets test suite. These are referenced as raw JPEGs in the
upstream ITKIsotropicWavelets remote module and must be republished as
CID content-links for the in-tree ingest into ITK proper (ITK issue
[#6160](InsightSoftwareConsortium/ITK#6160)).

The remaining five IsotropicWavelets fixtures are already published as
CID content-links in this repo (CID/bafkrei{f3k22q…,h6hdi2…,dmpadj…,ee4276…,dy2ng6…})
and require no changes.

| Filename | CID | sha256 | size (bytes) |
|---|---|---|---|
| `checkershadow_gray_540x420.jpg` | `bafkreighex2biwhd5bzvoxbc3fu7k4irmlch6cdjx5mzwua4isisx6xi4a` | `c725f41458e3e873575c22d969f5711162c47f0869bf599b501c44912bfae8e0` | 62509 |
| `checkershadow_illusion4full.jpg` | `bafkreid7i4fjtcxjhk6c6jobjgz4vpabjmyzcgx4sg75v53pamvbaun6iu` | `7f470a998ae93abc2f25c149b3cabc014b31911afc91bfdaf76f032a1051be45` | 181904 |

CIDs computed locally via IPFS CIDv1-raw (`hashlib.sha256` +
multihash prefix + base32-lower), matching every other blob in this repo.

Zero-byte markers per the pattern used in PR #50; IPFS pinning
(`w3 up`) will be performed by the maintainer prior to consumer use.

Author: hjmjohnson