fix: avoid repeated SDRAM allocator setup

[Old-Ding]

Summary

• prevent SDRAM.begin() from adding another global allocator block after one was already registered
• keep repeated calls with the same start address idempotent
• return failure when a later call tries to change the SDRAM malloc start address

Validation

• git diff --check
• inspected the SDRAM.begin() flow and the ea_malloc freelist setup to keep the guard at the single malloc_addblock() call site
• rg -n "malloc_addblock|malloc_start_address|int SDRAMClass::begin" libraries/Portenta_SDRAM/src libraries/ea_malloc -S

I did not run a full Arduino board build locally because the required board/toolchain setup is not provisioned here.

Fixes #1105

CI notes

• Current license/cla check is pending.
• This is an account/legal gate, not a code failure in the SDRAM allocator setup diff.

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