chore: untrack fuzz/target/ + gitignore subdirectory target/

Build artifacts from a local `cargo check` got included in the previous
commit because `.gitignore` only excluded `/target/` (root), not
subdirectory `target/` dirs. Removed the 173 stray files and added
`**/target/` so this can't recur for fuzz/, vendored crates, or any
other Rust subdir.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

.gitignore

@@ -81,3 +81,6 @@ htmlcov/
 # Crash recovery artifacts
 ai-cli-crash-capture/
 proofs/lean4/.lake/
+
+# Rust build outputs in subdirectories (e.g. fuzz/target/)
+**/target/