Claude review: rust: dma: add Coherent:init() and Coherent::init_with_attrs()

[Claude Code Review Bot <claude-review@example.com>]

Patch Review

Adds one-shot constructors that allocate uninitialized DMA memory, run an `Init` closure, and return the `Coherent` directly.

**Commit message refers to "CoherentInit"**: The message says "Compared to CoherentInit, Coherent::init() is a one-shot constructor" — but the type was renamed to `CoherentBox` in this v2 series (per the cover letter). The commit message should reference `CoherentBox` instead.

**Safety of init on uninitialized memory**: The `alloc_with_attrs` (without `__GFP_ZERO`) is used, so the memory is uninitialized. The safety comment says:
```rust
// - If `__init` fails, `self` is dropped, which safely frees the underlying `Coherent`'s
//   DMA memory. `T: AsBytes + FromBytes` ensures there are no complex `Drop` requirements
//   we are bypassing.
```
This is correct — `AsBytes + FromBytes` types have no drop glue that could observe uninitialized memory. However, if `__init` fails, the `Coherent` in `dmem` is dropped, calling `dma_free_attrs` on memory that was never initialized. This is fine since the DMA deallocation only cares about the address/size, not the content.

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