refactor: launch hashcrew as a new crate

[tisonkun]

Summary

• Rename the publishable package, source tree, imports, documentation, examples, benchmarks, tests, CI, and release tooling from rache to hashcrew, with a fresh 0.1.0 version and first-release changelog.
• Point crate metadata, workflow badges, semantic status metadata, and the local development remote at the renamed fast/hashcrew repository.
• Remove the empty integration-test library target and the unlinked HISTORY.md summary after verifying that Cargo still discovers every test target and ARCHITECTURE.md remains the single design reference.
• Remove Xxh3SecretTooShort::minimum_len() because it duplicated the canonical SECRET_SIZE_MIN constant, while retaining instance-specific actual_len().
• Make default and seeded Xxh3_64 and Xxh3_128 construction const, matching the other streaming states and compiling on Rust 1.85.

Design Notes

The ablation pass kept only changes with a demonstrated net benefit. An experiment to make the public XXH3 backend observer private was discarded before commit: target-specific dead-code handling or a more complex dispatch split cost more than the API reduction justified. Shared XXH3 streaming state, kernel tokens, Murmur 128-bit buffering, state metadata accessors, builders, and std::io::Write adapters remain because each removes real duplication or serves a distinct caller boundary. No runtime hashing implementation was changed.

Validation

• cargo x check
• cargo x lint
• cargo x test
• RUSTUP_TOOLCHAIN=1.85.0 cargo x test
• cargo x bench --no-run
• cargo bench --package benchmarks --bench throughput -- xxh3_64::hashcrew --min-time 0.01 --max-time 0.02
• RUSTFLAGS='-C target-feature=-neon' cargo +nightly miri test --package hashcrew --lib --no-default-features --release (scalar path; local Miri does not support the AArch64 kernel's inline assembly barrier)
• cargo package --package hashcrew --locked