Add `SDRAM.realloc()`

`SDRAM` currently supports `malloc()` and `free()`, but doesn't support `realloc()`.
Would it be possible to add it?

Start by locating the SDRAM implementation and reading the existing malloc() and free() paths to understand the current allocation contract. Determine the expected realloc() behavior for that implementation, then verify that SDRAM can resize allocations while preserving the required data and existing allocation behavior.

Written by the indexing model from the issue text.