feat: add Acoustic-2 measurement harness

[matveyb9]

Summary

• adds Acoustic-2 as a separate experimental measurement layer around the existing Acoustic-1 PCM/WAV decoder; it does not alter ADLP or Acoustic-1 framing
• defines bounded deterministic integer PCM transforms: leading silence, attenuation, seeded additive noise, hard clipping and periodic sample deletion
• reports only codec-observable inputs/outputs, dropped samples, bounded acquisition offset, consumed samples and ADLP profile after a successful decode
• adds a golden measurement contract over the committed Acoustic-1 fixture and CLI command measure-acoustic1
• adds bounded acceptance/rejection regression cases and keeps the unmodified Rust bridge/UI behavior intact
• adds English-canonical/Russian Acoustic-2 specification, research note, guides, platform status corrections, troubleshooting, public site status and CI bilingual-pair coverage

Validation

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace
• cargo run -p adlp-cli -- measure-acoustic1 ... using the committed Acoustic-1 fixture
• dart format --output=none --set-exit-if-changed lib test
• flutter analyze
• flutter test
• flutter build web --release
• flutter build linux --debug
• local static-site and bilingual document-pair validation

Scope boundary

Acoustic-2 is a reproducible synthetic PCM measurement harness only. Its values are not BER, SNR, range, room-noise, timing-recovery, clock-drift, microphone/speaker, Bluetooth, radio or device-compatibility evidence. No live audio adapter is introduced.