Claude review: dma-buf: heaps: system: document system_cc_shared heap

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

Patch Review

**Placement**: Correct -- the new entry is inserted between the `system` heap and the `default_cma_region` heap, which is the natural ordering since `system_cc_shared` is a variant of the system heap.

**Content accuracy**: The description covers the key aspects:
- What it allocates (virtually contiguous, cacheable, shared/decrypted memory)
- When it's present (CoCo VMs with memory encryption active)
- Why it exists (device DMA without TDISP support)
- When it's absent (non-CoCo configurations)

**Minor wording nit**: The phrase:

```
making them accessible for device DMA without TDISP
support
```

is slightly ambiguous -- it could be parsed as "accessible for [device DMA without TDISP support]" or "accessible for device DMA, [without needing TDISP support]". Both readings arrive at the same meaning, but adding a comma would make it unambiguous:

```
making them accessible for device DMA, without requiring TDISP support.
```

This is a very minor readability suggestion, not a blocking issue.

**Naming convention**: The name `system_cc_shared` aligns well with the naming convention guidelines documented later in the same file -- it describes the memory properties (shared/decrypted for confidential computing) rather than implementation details.

**Reviewed-by**: Already has `Reviewed-by: T.J.Mercier <tjmercier@google.com>`.

**Overall**: Clean documentation patch. No objections.

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Generated by Claude Code Patch Reviewer