Remove Ethash and Pearl backends; keep Equihash 192,7 only

Drop the non-Equihash algorithms and their integration points:
- delete src/ethash.rs + src/ethash/ and src/pearl.rs + src/pearl/
- remove the ethash/pearl/pearl-cuda features and pearl-only deps
  (blake3, primitive-types, rand, bincode, base64) from Cargo.toml
- drop the --algo flag and the pearl/ethash dispatch branches in main.rs
- remove the pearl-cuda NVRTC linking from build.rs
- drop the stale /pearl-dump/ .gitignore entry

Builds check cleanly with default features and --no-default-features.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitignore

@@ -11,9 +11,7 @@
 /mine.toml
 
 # Reverse-engineering scratch & competitor binaries — kept local, not committed.
-# Not part of the miner; pearl-dump also holds a 108 MB file over GitHub's limit.
 /alpha-miner
-/pearl-dump/
 
 # OS cruft
 .DS_Store