Improve typewriter audio for v0.3.2

[andy5090]

What changed

• Replaced synthesized typewriter sounds with compact CC0 Hermes Precisa 305 recordings.
• Added four real key-strike variants per profile with restrained pitch, level, brightness, and decay variation.
• Added non-repeating variant selection, low-frequency regression coverage, source hash verification, and third-party license documentation.
• Prepared version 0.3.2 for release.

Why

The previous synthesized sounds were overly resonant on MacBook speakers and repeated the same sample for every keystroke. The new recordings sound more natural while remaining compact and legally redistributable.

Validation

• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test --all-features (80 tests)
• cargo build --release

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