Implement Technical Deep Dives: memory allocators, lock-free structures, DMA, IRQ handlers, error propagation

[Copilot]

Implements the five unchecked Technical Deep Dives items from ROADMAP.md as annotated code snippets plus a unified documentation chapter.

New code snippets

File	Covers
research/snippets/memory/memory_allocator.rs	GFP flags, Box/Vec RAII over kmalloc, slab cache pattern, vmalloc
research/snippets/concurrency/lock_free.rs	Atomic counter, Treiber stack (CAS), arc-style refcount, RCU reader/writer
research/snippets/drivers/dma_operations.rs	Coherent/streaming DMA, direction as const-generic, MMIO volatile ops, #[repr(C)] descriptor ring
research/snippets/drivers/interrupt_handler.rs	IRQ return type, top-half/bottom-half split, threaded IRQ, per-device atomic state
research/snippets/error/error_propagation.rs	? propagation, C errno conversion, map_err, Option↔Result, RAII cleanup on error

Each file includes safety-documented unsafe blocks, in-file tests, and a pattern summary comment block. Example from error_propagation.rs:

// C — must check every return value manually; easy to silently ignore errors
int ret = step_one();
if (ret < 0) return ret;
ret = step_two();
if (ret < 0) return ret;

// Rust — ? propagates on failure; error paths are statically enforced
fn init() -> Result<()> {
    step_one()?;
    step_two()?;
    Ok(())
}

New documentation

• docs/09-technical-deep-dives.md — single-page reference covering all five topics: concept explanation, kernel API mapping, Rust idiom comparison, and common-bug/mitigation tables.

Updated files

• research/snippets/INDEX.md — entries for all new snippets; updated coverage checklist and learning path.
• ROADMAP.md — Technical Deep Dives items marked complete.

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