feat: rebuild heap4 card around upstream FreeRTOS allocator

UPSTREAM.md

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+# Upstream contract
+
+- Project: FreeRTOS-Kernel
+- Version: V11.3.0
+- Commit: `9b777ae5c5b8e9e456065a00294d1e5f5f9facf5`
+- Source used by Task 3: `portable/MemMang/heap_4.c`
+- License: MIT (as shipped in the pinned upstream checkout)
+
+RP2350 needs a port that is newer than the V11.3.0 tag:
+
+- Project: Raspberry Pi fork of FreeRTOS-Kernel
+- Commit: `4f7299d6ea746b27a9dd19e87af568e34bd65b15`
+- Files used on RP2350: matching kernel core plus
+  `portable/ThirdParty/GCC/RP2350_RISC-V`
+- Allocator used on RP2350: still the stable V11.3.0 `heap_4.c` above,
+  compiled instead of the fork's development-branch allocator.
+
+The canonical containers provide `/opt/FreeRTOS-Kernel`; the RP2350 profile
+also provides `/opt/FreeRTOS-RP2350`. The card carries no allocator copy.