test: simplify issue-15 regression to in-tree single file

Cleanup of the issue-15 branch before merge to main:

- Replace the corpus-walking regression test with a single in-tree
  file test pinned to the smallest reproducer (akfcrc022_e9_d3.0.jxl,
  22 KB). The full historical corpus (3 distinct-variant files)
  lives in imazen/codec-corpus PR #5 under jxl/conformance/ and is
  exercised by the existing codec_corpus parity tests when the
  corpus is reachable.
- Remove hard-coded /mnt/v/fuzzes/... path. Path now resolves via
  CARGO_MANIFEST_DIR -- CI-portable, no external setup required.
- Drop benchmark TSVs and the issue-15 sweep summary md from this
  branch; they were investigation scratch and the conclusions are
  captured in the upstream PR + tracking issues.

Test fingerprints the bug: decode succeeds, output is 512x512x{3,4}u8.
Pixel parity is verified by the codec_corpus tests against the
codec-corpus reference outputs once that PR merges.

Refs:
- libjxl/jxl-rs#765 (bug)
- libjxl/jxl-rs#766 (fix)
- imazen/codec-corpus#5 (full corpus)

Author: lilith