Migrate to dmdrvi 2.0 (64-bit I/O contract)

[JaxCalder]

Summary

• Bumps every DIF registration in src/dmfmc.c (_create, _free, _open, _close, _read, _write, _ioctl, _flush, _stat) from dmod_dmdrvi_dif_api_declaration(1.0, ...) to 2.0, matching the coordinated ecosystem-wide cutover in [dmsdio 3/9] Migrate all existing dmdrvi implementations to API 2.0 dmod-ecosystem#8.
• dmfmc_dmdrvi_read()/dmfmc_dmdrvi_write(): offset changed from uint32_t to dmdrvi_offset_t (signed 64-bit) and the return type changed from size_t to dmdrvi_ssize_t, with explicit -EINVAL (negative offset) and -EOVERFLOW (size > INT64_MAX) guards added. The existing bounds-check against context->result.memory_size_bytes was adapted to compare via dmdrvi_size_t instead of the driver's own uint32_t port-layer size field, so a large 64-bit offset can no longer silently truncate and pass a bounds check it shouldn't.
• dmfmc_dmdrvi_stat()'s stat->size = context->result.memory_size_bytes assignment is now a safe widening uint32_t -> dmdrvi_size_t; no cast was needed or added.
• Pre-existing bug fixed: dmfmc_dmdrvi_open() was declared as (dmdrvi_context_t context, int flags), missing the dev_num parameter that both the 1.0 and 2.0 DIF contract require. It was being called through a 3-argument function pointer while only declaring 2 parameters - undefined behavior that predated this cutover, the same class of bug found and fixed in dmclk's earlier 2.0 migration (see Migrate to dmdrvi 2.0 (64-bit I/O contract) dmclk#29). Fixed by adding the dev_num parameter (unused, since dmfmc exposes a single device per context) with a (void)dev_num cast and doc comment, matching the dmclk pattern.
• Added/updated doc comments (@param/@return) for _open, _read, _write to describe the new signature and types.

Test plan

• Local build against dmfmc's actual supported target (its CMakeLists.txt/config.cmake hard-code DMOD_TOOLS_NAME=arch/armv7/cortex-m7 for the default DMFMC_MCU_SERIES=stm32f7, matching its CI matrix - dmfmc has no x86_64 port since it drives a real STM32 memory controller): cmake -S . -B /tmp/build_dmfmc_test -DDMOD_MODULE_VERSION=0.0.1 && cmake --build /tmp/build_dmfmc_test --target dmfmc completed with [100%] Built target dmfmc, no new warnings, against a freshly-fetched dmdrvi 2.0 (verified dmod_dmdrvi_dif(2.0, ...) in the fetched dmdrvi.h).
• A maintainer should validate on real STM32F7/F4 hardware with SDRAM attached, particularly the fixed _open signature (confirm callers now pass dev_num correctly) and the _read/_write bounds-check behavior at large offsets.

Part of choco-technologies/dmod-ecosystem#8.