chore: prepare v0.1.2 release

[tisonkun]

Summary

Prepare the 0.1.2 patch release from v0.1.1, including improved short-chunk XXH64 streaming and complete upstream source licensing. Public APIs, hash outputs, features, and MSRV remain unchanged.

• Bump hashcrew and its lockfile entry to 0.1.2 and finalize the dated changelog.
• Select stable once for the release session, use cargo x build --locked to compile benchmark targets, and document releasing the prepared manifest version with cargo release --package hashcrew --execute.
• Select the packaged crate by Cargo's current version and target directory so cached directories from older releases cannot break package tests.
• Align the README's MSRV policy with Fast Template and document toolchain selection: workflows inherit the active toolchain, Clippy and rustfmt explicitly use nightly, and cargo +1.85.0 x test overrides stable for the MSRV check.

The seeded XXH3 const-API investigation remains tracked in #17 for a future compatibility decision.

Validation: cargo x lint (Clippy and rustfmt on nightly); stable feature checks and workspace build; cargo x test on stable and Rust 1.85.0; the documented throughput and streaming release-size benchmarks; and cargo release --package hashcrew dry run with stable selected. The clean 0.1.2 archive contains the complete license and inherited README. Packaged tests pass with and without std, including when 0.1.0 and 0.1.1 directories remain in the build cache.