no-std Step 9 — CI job + smoke-test for `no_std + alloc` compile verification

[Mec-iS]

Part of #413 — feat: no-std feature flag for embedded/edge targets

Task

Add a CI job that proves smartcore builds on a real no_std + alloc target and does not silently re-acquire std coupling in future PRs.

Minimum viable CI (recommended first pass)

A compile-check against thumbv7m-none-eabi (ARM Cortex-M3, no OS, no std):

# .github/workflows/no-std.yml
name: no-std compile check
on: [push, pull_request]
jobs:
  no-std:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: dtolnay/rust-toolchain@stable
        with:
          targets: thumbv7m-none-eabi
      - name: cargo check (no-std + alloc)
        run: cargo check --no-default-features --target thumbv7m-none-eabi

This requires no QEMU, no linker, no #[panic_handler] in the library crate itself — cargo check only verifies the dependency graph and type-checks.

Smoke-test example (stretch goal)

A minimal examples/no_std_knn.rs (or examples/no_std_logistic.rs) that runs KNN or LogisticRegression under QEMU-emulated ARM:

#![no_std]
#![no_main]
extern crate alloc;
use smartcore::neighbors::knn_classifier::KNNClassifier;
// ... minimal fit/predict with static data

Full QEMU run adds ~2–3 hrs of CI configuration (linker, arm-none-eabi-gcc or lld, #[panic_handler], QEMU install). Recommended as a follow-up once the compile-check gate is green.

Acceptance

• A new GitHub Actions workflow file (e.g. .github/workflows/no-std.yml) runs cargo check --no-default-features --target thumbv7m-none-eabi on every push and PR
• The workflow is green on main after all Steps 1–8 are merged
• (Stretch) A smoke-test example compiles and runs on QEMU-emulated ARM in CI
• Docs note the no_std path in the user guide or a new "Embedded / no-std" section

Estimated effort

• Compile-check CI only: ~1–1.5 hrs
• Full QEMU run: ~2–3 hrs additional

Dependencies

Blocked on Steps 1–8 all being merged. This is the final integration gate.