Implement cursor reallocation handling in DynamicBuffer pinning example

[Copilot]

The DynamicBuffer example in the PhantomPinned documentation had a TODO noting that Vec::push may reallocate, invalidating the NonNull<u8> cursor pointer.

Changes

• Calculate cursor offset before push: Store the cursor position as an offset from the data buffer start using offset_from()
• Update cursor after push: Reconstruct the cursor pointer at the same logical offset in the potentially reallocated buffer using offset()
• Add explanatory comments: Document why the offset calculation is necessary for correctness

pub fn push(&mut self, byte: u8) {
    // Calculate the cursor offset before the push (which may reallocate)
    let offset = unsafe {
        self.cursor.as_ptr().offset_from(self.data.as_ptr())
    };
    
    self.data.push(byte);

    // Update cursor to point to the same offset in the (potentially new) buffer
    self.cursor = unsafe {
        NonNull::new_unchecked(self.data.as_mut_ptr().offset(offset))
    };
}

This demonstrates the correct pattern for maintaining a pointer into a dynamically-sized container, which is essential context for understanding when pinning is necessary.

Original prompt

Work on TODO: self.data may have reallocated; ensure that th cursorpoints the the correct place (from src/unsafe-deep-dive/pinning/phantompinned.md)

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