Scaffold Cargo workspace with post-01 placeholder crate

Initial structure for the Rust via Project Euler series:
- Cargo workspace with posts/post-01-multiples-of-3-and-5/ as the first crate
- Three binaries (beginner/medium/expert) wired to a shared lib with
  placeholder solutions; Post 1 will replace them with the real three-level walk
- criterion bench harness + correctness test verifying the Euler answer (233168)
- rust-toolchain.toml pinning to rustc 1.95.0 for reproducible numbers
- GitHub Actions CI: fmt, clippy (warnings as errors), tests, bench compile
- results.json stub recording benchmark hardware (Apple M5, 10c, 24GB, macOS 26.2)
- Dual MIT/Apache-2.0 license, README explaining reproduction flow

Author: xTMNTxRaphaelx

.github/workflows/ci.yml

@@ -0,0 +1,34 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+env:
+  CARGO_TERM_COLOR: always
+  RUSTFLAGS: -D warnings
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install pinned toolchain (rust-toolchain.toml)
+        run: rustup show active-toolchain || rustup toolchain install
+
+      - uses: Swatinem/rust-cache@v2
+
+      - name: Check formatting
+        run: cargo fmt --all -- --check
+
+      - name: Clippy
+        run: cargo clippy --workspace --all-targets --all-features -- -D warnings
+
+      - name: Tests
+        run: cargo test --workspace --all-targets
+
+      - name: Compile benches (CI numbers are not canonical — local runs populate results.json)
+        run: cargo bench --workspace --no-run

.gitignore

@@ -0,0 +1,5 @@
+/target
+.DS_Store
+*.swp
+/.idea/
+/.vscode/