riscv64: PLIC base address DTB-discovered, QEMU-virt constant as fallback (§V.3 item 3)

plic_init() now takes boot_info->dtb (threaded through apic_init()) and
tries fdt_find_node_by_compatible(dtb, "sifive,plic-1.0.0") ->
fdt_find_prop_in_node(..., "reg", ...) before falling back to the
QEMU-virt-specific constant it previously hardcoded unconditionally.
Reuses the node-scoped DTB lookup primitive built for the aarch64 GIC
base fix unchanged. s_plic_base is now a runtime uintptr_t, same shape
as apic.c's s_gicd_base/s_gicc_base.

This system's QEMU/UEFI riscv64 firmware does not forward a DTB to the
guest (timer.c's own timebase-frequency read falls back too, confirmed
in this boot's own log), so only the no-DTB fallback branch is exercised
here -- the success branch (a real DTB with a matching PLIC node) stays
unverified until real Milk-V Mars hardware. FABRIC-3.md's first-drafted
claim that the success branch would run (based on a stale comment in
plic.c's own pre-fix header) was checked against the actual log and
corrected before this commit.

3-arch acceptance: amd64/aarch64 don't compile these files, so their
runs are non-regression on untouched files only. riscv64's own boot log
confirms the fallback path prints exactly as designed and boot reaches
zuse)ok> unchanged.

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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