no-std Step 3: decouple `datasets` feature from `std_rand` in Cargo.toml

[Mec-iS]

Sub-task of #413.

What

Remove the implicit std_rand dependency from the datasets feature in Cargo.toml:

# Before
datasets = ["std_rand", "serde"]

# After
datasets = ["serde"]

The dataset bytes are statically embedded in the binary (serialized bundles), so they have no runtime RNG dependency. The std_rand pull was incidental, not required.

Why

Allows toy datasets (iris, boston, digits, etc.) to be available on embedded targets without dragging in rand/std and the OS RNG.

Notes

• Verify all dataset loading functions compile and pass tests with cargo test --features datasets (without std_rand).
• If any dataset codepath uses rand, document it and decide whether to gate it or replace with a seeded SmallRng.
• Update CHANGELOG.md and any feature documentation.

Acceptance

• cargo build --no-default-features --features datasets compiles
• cargo test --features datasets passes
• cargo test --features "datasets,std_rand" still passes (no regression)

Estimated effort: ~45 min