Gate experimental features for v0.1.0

[tamirelazar]

Summary

• Multi-species CLI/help/overlay surface behind new multi-species cargo feature (input-boundary accessors; sim core and render internals unchanged)
• Choir flags, F2 key, and overlay hint moved fully under the audio feature (they leaked into the default build, non-functional)
• Dither marked dev-only: flags hidden but functional, runtime keys locked unless enabled at startup, gray "(dev)" in overlays
• Fixed vacuous dither golden test (invalid flag, 0-byte golden)
• CI: cargo test --all-features --lib on Linux so gated code can't break silently
• README scrubbed; Experimental section with tracking issues

Test plan

• cargo test + cargo test --all-features --lib
• clippy default + all-features, -D warnings
• --help clean of species/choir/dither
• --features multi-species / --features audio builds work