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

boot_media/rpi5/README.md

@@ -0,0 +1,12 @@
+# Raspberry Pi 5 native boot media
+
+What goes on the SD card for the native (non-UEFI) boot path (FABRIC-3.md §IV):
+
+- `config.txt` — checked in here, done (§IV.3 item 6).
+- `bcm2712-rpi-5-b.dtb` — Raspberry Pi firmware's own stock DTB; not built by this
+  project, copied from the firmware release the card is otherwise built from.
+- `kernel_2712.img` — **does not exist yet.** `config.txt` names it, but no build target
+  in `Makefile.starkernel` currently emits a raw image by this name at load address
+  0x80000 — item 1 (entry stub) explicitly scoped a separate-image build target as future
+  work, not part of that item. Building this file is the remaining prerequisite before
+  item 7 (assembling the card) is possible.

boot_media/rpi5/config.txt

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+# LithosAnanke native (non-UEFI) boot config for Raspberry Pi 5 (BCM2712).
+# FABRIC-3.md §IV.3 item 6 -- see that section's own doc comment for the
+# research trail behind each line (confirmed against raspberrypi.com's
+# current config.txt reference and a real, working Pi 5 bare-metal
+# project's own checked-in config.txt, not assumed from general Pi
+# knowledge).
+
+# Load our own kernel image directly -- no UEFI, no Linux. kernel_2712.img
+# is the real filename Pi 5/500/CM5 firmware looks for first (its own
+# 16K-page-size build); naming our own image this exactly, rather than
+# falling back to kernel8.img, removes any ambiguity about which file the
+# firmware picks up.
+kernel=kernel_2712.img
+
+# Disable the firmware's Linux-compatible-image sanity check. Without
+# this, official docs describe it as checking for "a compatible Device
+# Tree file before attempting to boot" and warn that "older non-compatible
+# kernels would be loaded and then hang" -- documented specifically as the
+# bare-metal-development escape hatch. One real working Pi 5 bare-metal
+# project's own config.txt does NOT set this and still boots, so it may
+# not be strictly required for every non-Linux image; kept here anyway
+# since the official doc names this exact use case and there is no
+# hardware in hand yet to verify which behavior is actually gated.
+os_check=0
+
+# Pin the exact devicetree explicitly rather than relying on the
+# firmware's own board-revision auto-selection (which would also resolve
+# to this file on real Pi 5 hardware, per general config.txt convention --
+# explicit is certain, not just probably-correct).
+device_tree=bcm2712-rpi-5-b.dtb
+
+# Enable the dedicated debug UART (Pi 5's own JST connector, not the GPIO
+# 14/15 header pins). Not because serial capture hardware is set up for
+# this bring-up (that decision is HDMI-only, FABRIC-3.md §IV) -- this is
+# a firmware-side switch that costs nothing when nothing is listening, and
+# it is the only diagnostic channel that survives a framebuffer failure.
+# rpi5_uart_base() (rpi5_dtb.c) already exists and is currently uncalled;
+# this is the hook a UART-based fallback console would use if the mailbox
+# framebuffer driver's two documented unverified details turn out wrong on
+# real hardware.
+enable_uart=1
+
+# Deliberately NOT set, researched but not included this pass:
+#   dtoverlay=uart0   -- GPIO 14/15 header UART; needs a USB-serial
+#                        adapter this bring-up doesn't have (HDMI-only
+#                        decision). enable_uart=1 above covers the JST
+#                        debug UART instead, which needs no extra cable.
+#   dtparam=pciex1    -- one real Pi 5 bare-metal project's own config.txt
+#                        comments this as "Required for RP1 access", which
+#                        would be surprising (RP1 is always-on in stock Pi
+#                        5 boots) and is community-reported, not confirmed
+#                        against official docs. Not needed for this boot
+#                        (no RP1 access yet -- FABRIC-3.md §IV.3 item 5 is
+#                        still re-scoped, not implemented). Revisit if that
+#                        item's own real Broadcom-indirect PCIe work needs
+#                        it.
+#   arm_64bit=1       -- researched and dropped: official docs state
+#                        "Models that only support a 64-bit kernel ignore
+#                        this flag" -- inert on Pi 5, not wrong to include,
+#                        just meaningless. The original item 6 wording
+#                        named this; corrected here rather than carried
+#                        forward.