FABRIC-3.md: pci_init() DTB path item was wrong -- re-scope, don't build the ECAM version

Investigated before writing code, per this document's own discipline, and
found the original item 5 text's assumption doesn't hold: it framed this
as "swap the ACPI MCFG lookup for a DTB one, same flat-ECAM access
underneath." Confirmed against three primary sources that this is wrong:

- bcm2712-rpi-5-b.dts: RP1 sits under pcie2 ("brcm,bcm2712-pcie"),
  reg = <0x10 0x00120000 0x00 0x9310> -- a ~37KB window, far too small
  for a flat 256MB ECAM region.
- pcie-brcmstb.c (the real Linux driver for this compatible string):
  brcm_pcie_map_bus() computes a standard ECAM-shaped offset but accesses
  it through an IDX_ADDR/DATA_ADDR indirect index+window pair, not a
  direct MMIO read. A real third config-access mechanism, not a bigger
  version of the DTB-lookup task.

Re-scoped item 5 (§IV.3) to record the real shape: pci.c's cfg_read32/
cfg_write32 family dispatches today via #ifdef ARCH_AMD64 vs. everything
else (flat ECAM) -- but QEMU aarch64 (flat ECAM, real ACPI MCFG) and Pi 5
aarch64 (Broadcom indirect windowing) are the same build, so this needs a
runtime dispatch change to a file all three architectures currently boot
through, not a small addition. Also flagged: pci_init()'s vmm_map_range()
for the ECAM window is amd64-only today (aarch64/riscv64 rely on UEFI's
identity map); native boot has no UEFI identity map at all, so any PCIe
work here needs its own explicit mapping regardless of access mechanism.

Also corrected §V.3 item 5's now-stale "shares the same code" cross-
reference -- the Mars's JH7110 PCIe controller needs its own primary-
source check, not an assumed-shared implementation with RP1's.

Not implemented this pass -- recording the real shape is the deliverable;
building the runtime-dispatch version is a materially larger, separate
task. Doc-only change; 3-arch boot verified per the project's standing
rule, proving only that nothing regressed.

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

capsules/BLOCK_MAP.md

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+<!-- Generated by mkcapsule --manifest  2026-09-05T02:45:26Z -->
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