Claude review: rust: ptr: remove implicit index projection syntax

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

Patch Review

Removes the backward-compatibility shims added in patch 3, now that all callers have been converted:

From `projection.rs`:
```rust
-    // For compatibility
-    (@gen $ptr:ident, [$index:expr]? $($rest:tt)*) => {
-        $crate::ptr::project!(@gen $ptr, [try: $index] $($rest)*)
-    };
-    (@gen $ptr:ident, [$index:expr] $($rest:tt)*) => {
-        $crate::ptr::project!(@gen $ptr, [build: $index] $($rest)*)
-    };
```

And from `dma.rs`, the corresponding old-syntax parse arms in `dma_write!`:
```rust
-    (@parse [$dma:expr] [$($proj:tt)*] [[$index:expr]? $($rest:tt)*]) => {
-        $crate::dma_write!(@parse [$dma] [$($proj)* [$index]?] [$($rest)*])
-    };
-    (@parse [$dma:expr] [$($proj:tt)*] [[$index:expr] $($rest:tt)*]) => {
-        $crate::dma_write!(@parse [$dma] [$($proj)* [$index]] [$($rest)*])
-    };
```

Clean removal. The `dma_read!` macro (not shown in the diff) presumably also had similar old-syntax arms -- it's worth verifying that `dma_read!` is also cleaned up. Looking at the diff for `dma.rs` in this patch, only `dma_write!` is modified. The `dma_read!` macro delegates to `project_pointer!` internally, so it would go through `ptr::project!(@gen ...)` which no longer has the old-syntax arms. If `dma_read!` had its own parsing arms for `[$index]?` and `[$index]`, those should be removed too. However, looking at the overall diffstat, the `dma_read!` macro likely delegates parsing to `project_pointer!` directly (unlike `dma_write!` which has its own `@parse` arms), so this may be correct as-is.

Already has Reviewed-by from Alexandre, Andreas, and Alice. No issues with the removal itself.

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