riscv64: PLIC bring-up, external-interrupt substrate (item 4.3.5b)

Punch list §25 item 4.3.5b complete.

sie.SEIE enabled, PLIC threshold/claim/complete wired into the trap
handler. Verified with a UART-loopback synthetic interrupt (PLIC has no
software set-pending register, unlike GICv2): claim_count=1, last_irq=10,
IIR confirms genuine receive-data cause, byte matched exactly. Self-test
code run once for evidence then fully reverted, per Captain Bob's ruling;
only the permanent substrate remains, no source enabled by default.
Three-arch acceptance boot clean, zero exceptions.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/starkernel/arch/riscv64/apic.c

@@ -6,14 +6,17 @@
 
 /**
  * apic.c (riscv64) - APIC stub. No Local APIC on RISC-V; interrupt
- * controller is PLIC (Platform-Level Interrupt Controller).  These stubs
- * satisfy the common kernel interface; PLIC wiring is a later milestone.
+ * controller is PLIC (Platform-Level Interrupt Controller). apic_init()
+ * calls into plic.c for the real PLIC bring-up (item 4.3.5b); apic_eoi()
+ * stays a no-op since PLIC completion happens per-source in the trap
+ * handler (claim/complete), not via a single shared EOI register.
  */
 
 #include "apic.h"
 #include "uefi.h"
 #include "console.h"
 #include "timer.h"
+#include "starkernel/plic.h"
 #include <stdint.h>
 
 static uint64_t s_timer_period_tsc = 0;
@@ -149,20 +152,23 @@ void riscv64_timer_rearm(void)
 }
 
 /*
- * @brief Initialise the interrupt controller (RISC-V PLIC stub).
+ * @brief Initialise the interrupt controller: PLIC bring-up (item 4.3.5b).
  *
  * On x86-64 this function configures the Local APIC. On RISC-V the
  * Platform-Level Interrupt Controller (PLIC) handles external interrupt
- * routing; PLIC bring-up is deferred to a later milestone. This stub
- * satisfies the common @c apic_init() call site in @c kernel_main()
- * and always returns success.
+ * routing -- @c plic_init() (arch/riscv64/plic.c) maps it, sets the S-mode
+ * context's threshold, and enables @c sie.SEIE. No source is enabled here;
+ * that is each future consumer's own job (4.3.5c for virtio-keyboard).
  *
- * @param boot_info Kernel boot information (ACPI/PLIC base address); unused.
+ * @param boot_info Kernel boot information; unused (PLIC base is a verified
+ *                  constant, not discovered from @c boot_info->dtb -- see
+ *                  plic.c's file header for why).
  * @return 0 always.
  */
 int apic_init(BootInfo *boot_info)
 {
     (void)boot_info;
+    plic_init();
     return 0;
 }