feat: ParallelConcurrency — auto-size concurrency from CPU cores

.hermes/plans/mytheclipse-round20-spec.md

@@ -0,0 +1,35 @@
+# Implementation Spec: Round 20 — Auto Concurrency
+
+## Goal
+`parallel_map` / `parallel_map_unordered` / `parallel_for_each` terima
+`usize` (eksplisit, existing) ATAU `()` (auto dari host CPU cores). Tidak
+perlu nama API baru — trait `ParallelConcurrency` resolve di call-site.
+
+## Design
+- Trait `ParallelConcurrency`: `fn resolve(self) -> usize`
+  - impl `usize` → `self.max(1)` (behavior lama, backward compatible)
+  - impl `()` → `std::thread::available_parallelism()` fallback 1
+- 3 fungsi berubah: `concurrency: usize` → `concurrency: C where C: ParallelConcurrency`
+  - `let n = concurrency.resolve();`
+  - Body tidak berubah (pakai `n`)
+- Export trait di lib.rs
+
+## Backward compat
+Caller existing `parallel_map(items, 4, f)` tetap compile — `4` resolve ke
+`usize` (satu-satunya impl integer). Literal inference OK karena trait bound
+memaksa `usize`.
+
+## Files
+- crates/mytheclipse/src/parallel_map.rs (trait + 3 signature)
+- crates/mytheclipse/src/lib.rs (export ParallelConcurrency)
+- crates/mytheclipse/examples/scaling_demo.rs (demo auto run)
+- doctests: tambah contoh auto `()` di parallel_map & parallel_for_each
+- tests: `auto_concurrency_uses_cpu_cores` (peak ≤ cores), hasil benar
+
+## Verification
+1. `cargo test -p mytheclipse parallel --all-features` — 0 FAILED
+2. `cargo test -p mytheclipse --test race_stress --all-features` — 0 FAILED
+3. `cargo build --workspace --all-features` — exit 0
+4. `cargo clippy --workspace --all-features` — 0 new
+5. `cargo run --example scaling_demo` — auto run peak == cores
+6. spec + commit + push