Use cargo-zigbuild for cross-compilation, add build caching

- Integrate cargo-zigbuild as the default build backend, falling back
  to cargo build with best-effort linker detection if not installed
- Auto-install rustup targets when missing (rustup target add)
- Use persistent target/krust/ directory instead of temp dirs so
  incremental compilation works across runs
- Remove .cargo/config.toml from repo (add to .gitignore) since
  zigbuild handles cross-linker configuration
- Improve error messages: suggest cargo-zigbuild when linker not found
- Simplify CI to use zigbuild instead of per-target system linkers
- Simplify Makefile by removing setup-cross-compile/verify targets
- Update README and CLAUDE.md to reflect new approach

https://claude.ai/code/session_01EcsZDNeWn56wFqryb4Wq7r

.gitignore

@@ -16,6 +16,7 @@ Thumbs.db
 
 # Cargo
 Cargo.lock
+.cargo/
 
 # Coverage
 tarpaulin-report.html