fdt.c: node-scoped lookup extension (FABRIC-3.md SSIV.3/SSV.3 shared item)

Adds fdt_find_node_by_compatible() and fdt_find_prop_in_node() to the
minimal FDT reader -- the extension fdt.h's own header comment already
flagged as a known future need ("item 0.6 will need node-scoped reg
lookups"), now with real consumers: the Pi 5's UART/mailbox register
addresses (native boot, no ACPI) and the Milk-V Mars's real PLIC base
address (currently hardcoded to QEMU-virt's own value).

fdt_find_node_by_compatible() matches any entry in a node's
NUL-separated "compatible" list, first match in document order.
fdt_find_prop_in_node() scopes to that one node's own direct
properties only -- stops at the first child node or the node's own
end, per the DT spec's ordering guarantee that a node's properties
always precede its children. Same minimal, non-tree-building,
single-linear-scan-per-call style as the existing reader; no new
state, no allocation.

Compile-only verification -- no caller wired in yet, this is the
shared primitive both boards' own punch-list items will call once
built. Verified 3-arch boot to ok> (amd64/aarch64/riscv64, each in
the foreground).

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

include/starkernel/fdt.h

@@ -8,13 +8,20 @@
  * fdt.h - Minimal flattened-devicetree reader
  *
  * Just enough of the Devicetree Specification v0.4 §5 to pull values out of
- * the blob the UEFI firmware publishes under EFI_DTB_TABLE_GUID. Read-only,
- * no allocation, no tree construction — it walks the structure block each
- * call, which is fine for the handful of boot-time lookups the kernel needs.
+ * the blob the UEFI firmware publishes under EFI_DTB_TABLE_GUID, or that a
+ * native (non-UEFI) boot entry passes directly. Read-only, no allocation, no
+ * tree construction — it walks the structure block each call, which is fine
+ * for the handful of boot-time lookups the kernel needs.
  *
  * Deliberately not a general devicetree library. Added for punch-list item
- * 0.3 (riscv64 timebase-frequency); item 0.6 will need node-scoped `reg`
- * lookups for the aarch64 GIC and may extend this.
+ * 0.3 (riscv64 timebase-frequency); extended (FABRIC-3.md §IV.3/§V.3,
+ * 2026-09-04) with node-scoped lookup, for exactly the case this header
+ * originally flagged as a future need (item 0.6's aarch64 GIC) plus its
+ * real, concrete consumers as of this pass: the Raspberry Pi 5's UART/
+ * mailbox register addresses (native boot, no ACPI) and the Milk-V Mars's
+ * real PLIC base address (currently hardcoded to QEMU-virt's own value,
+ * `arch/riscv64/plic.c`'s own doc comment already warned this isn't
+ * assumed stable across configurations).
  */
 
 #ifndef STARKERNEL_FDT_H
@@ -62,4 +69,44 @@ const void* fdt_find_prop(const void* fdt, const char* name, uint32_t* len_out);
  */
 int fdt_prop_u32(const void* fdt, const char* name, uint32_t* out);
 
+/**
+ * @brief Find the first node whose "compatible" property matches @p compatible.
+ *
+ * "compatible" is a NUL-separated list of strings (DT spec §2.3.1) — matches
+ * if @p compatible equals any one entry in the list, not just the whole
+ * property verbatim. Scans the whole tree in document order; the first
+ * matching node wins if more than one exists.
+ *
+ * @param fdt         Blob, already checked with @c fdt_valid().
+ * @param compatible  Compatible string to match, NUL-terminated.
+ * @return An opaque handle to the matched node, for use with
+ *         @c fdt_find_prop_in_node() only (not a raw offset or a pointer
+ *         to anything else meaningful) — or NULL if no node matches.
+ */
+const void* fdt_find_node_by_compatible(const void* fdt, const char* compatible);
+
+/**
+ * @brief Find a property by name, scoped to one node.
+ *
+ * Like @c fdt_find_prop(), but scans only @p node's own direct properties
+ * (as returned by @c fdt_find_node_by_compatible()) — stops at the first
+ * child node or the end of @p node's property list, never descends into
+ * children, never continues into a sibling. This is the difference that
+ * matters for a property name like "reg", which is not unique across the
+ * tree the way "timebase-frequency" (the whole reason @c fdt_find_prop()
+ * was originally sufficient) happens to be.
+ *
+ * @param fdt      Blob, already checked with @c fdt_valid().
+ * @param node     Handle from @c fdt_find_node_by_compatible(); NULL is
+ *                 safe and returns NULL (propagates a failed node lookup
+ *                 without a separate caller-side check).
+ * @param name     Property name, NUL-terminated.
+ * @param len_out  Receives the property length in bytes; may be NULL.
+ * @return Pointer to the property value inside @p fdt, or NULL if not
+ *         found (or if @p node is NULL). The value is big-endian as
+ *         stored in the blob.
+ */
+const void* fdt_find_prop_in_node(const void* fdt, const void* node,
+                                   const char* name, uint32_t* len_out);
+
 #endif /* STARKERNEL_FDT_H */