boot_media/rpi5/config.txt: Pi 5 native boot config (FABRIC-3.md §IV.3 item 6)

Researched against the official raspberrypi.com config.txt reference and one
real, working Pi 5 bare-metal project's own checked-in config.txt
(leopoldch/BatMetal), not assumed from general Pi knowledge:

- kernel=kernel_2712.img, os_check=0, device_tree=bcm2712-rpi-5-b.dtb
- enable_uart=1 added (correction to the item's original text): costs
  nothing when unused, and is the only diagnostic channel that would survive
  a framebuffer failure on the untested mailbox path (rpi5_native_boot.c).
- arm_64bit=1 dropped (correction to the item's original text): confirmed
  inert on Pi 5 by official docs.
- dtparam=pciex1 researched, deliberately omitted with rationale recorded --
  community-reported only, not needed until item 5's re-scoped RP1/PCIe work.
- os_check=0 vs. BatMetal's own config (which omits it) recorded as an open,
  unresolved discrepancy rather than silently picked.

boot_media/rpi5/README.md notes the one remaining gap this exposes:
kernel_2712.img names a file nothing currently builds (item 1's
separate-image build target is still outstanding future work).

Doc-and-config only, no compiled code changed. Three-arch QEMU acceptance
(amd64/aarch64/riscv64, in order) run to confirm non-regression only.

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

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