feat(v0.2): Rust memory pool, scheduler, and tiled matmul

[m96-chan]

Summary

PyGPUkit v0.2 major update with Rust backend for high-performance memory management and task scheduling.

New Features

• Rust Memory Pool: LRU eviction, size-class allocator (13x faster than Python)
• Rust Scheduler: Task state machine with memory tracking (5x faster than Python)
• Tiled Matmul: CUDA kernel with shared memory (RTX 30XX+)
• Python Thin Wrappers: Full backward compatibility via PYGPUKIT_USE_RUST env var

Benchmark Results

Rust vs Python Backend

Operation	Rust	Python	Speedup
Memory Pool (10K allocs)	3.05ms	40.97ms	13.4x
Scheduler (10K submits)	9.27ms	44.56ms	4.8x

Matmul Performance (RTX 3090 Ti)

Size	GFLOPS
1024x1024	~1410
2048x2048	~1918
4096x4096	~2091

Architecture

PyGPUkit/
├── rust/
│   ├── pygpukit-core/     # Pure Rust (memory, scheduler)
│   └── pygpukit-python/   # PyO3 bindings
├── native/                # C++ CUDA backend
└── src/pygpukit/          # Python thin wrappers

Changes

Rust Implementation (rust/)

• pygpukit-core: Memory pool with LRU eviction and size-class allocator
• pygpukit-core: Scheduler with task state machine and memory tracking
• pygpukit-python: PyO3 bindings (MemoryPool, Scheduler, TaskMeta, etc.)

Python Thin Wrappers

• MemoryPool delegates to Rust backend
• Scheduler delegates to Rust backend
• PYGPUKIT_USE_RUST=0 to disable Rust (fallback to pure Python)

CI/CD Integration

• release.yml builds Rust module with maturin
• Single wheel contains both C++ and Rust extensions
• pip install pygpukit installs everything

Test plan

• All 124 tests pass
• Memory pool: allocation, free, reuse, eviction, restore
• Scheduler: submit, step, memory tracking, completion
• Matmul: tile-aligned/non-aligned sizes, rectangular matrices
• Thread safety tests

Usage

from pygpukit.memory import MemoryPool
from pygpukit.scheduler import Scheduler

# Automatically uses Rust backend (13x faster)
pool = MemoryPool(1024 * 1024 * 100)  # 100 MB
scheduler = Scheduler(total_memory=pool.quota)

# Disable Rust (for debugging)
# export PYGPUKIT_USE_RUST=0

Closes #26

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