FABRIC-3.md §IV.3 item 5: pci_init() DTB path re-scoped, doc-only, nothing implementable this pass

Re-examined the Pi 5 RP1 PCIe access re-scoping (§IV.3 item 5) to see what
was actually buildable before real hardware arrives (2026-09-17), and
found nothing safely implementable this pass -- recorded three corrections:

1. The "vmm_map_range() needed for native boot" secondary finding is
   wrong as stated: arch_mmu_init() is a stub on both non-amd64 ISAs,
   vmm.c's load_cr3() is a no-op outside __x86_64__, and riscv64's satp
   is explicitly left at Bare (this session's own §V.3 item 7 fix). No
   non-amd64 ISA ever turns its own MMU on today, so VA==PA and there is
   nothing to map. Reframed as a future concern conditional on MMU
   bring-up, not a current prerequisite.

2. No silent-wrong-answer risk exists today: with no ACPI on native Pi 5
   boot, pci_init()'s MCFG parse already fails cleanly and aarch64 takes
   no ECAM fallback, rather than misreading RP1's 37 KB indirect window
   as flat ECAM.

3. The "make config-space dispatch runtime instead of compile-time" half
   of the re-scoped fix, considered on its own, isn't separable:
   portio_read32() and siblings use outl/inl inline asm that cannot exist
   in an aarch64/riscv64 translation unit, so any case referencing them
   stays #ifdef-gated regardless -- an enum wearing the same compile-time
   selection. It only becomes real dispatch once a second non-amd64
   mechanism is actually compiled in.

Broadcom indirect access itself remains unimplemented: no QEMU model
exists for "brcm,bcm2712-pcie" (zero branches would ever execute before
real hardware, unlike the guard-exercised GIC/PLIC DTB fixes), and
config-space access alone is insufficient for RP1 to enumerate without
the real driver's controller bring-up (link training, PERST, window
setup) -- building only the index/data window would compile and boot
while silently never working.

No code changed. 3-arch acceptance run anyway per convention: all three
reach zuse)ok> with identical virtio-blk/xHCI PCI device discovery to
the pre-change baseline (commit 9b6de5d), confirming no regression from
a change that touched no source.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019YcT3H2PQeyujrzjqS3Var

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