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>

include/starkernel/plic.h

@@ -0,0 +1,53 @@
+/*
+  StarForth — Steady-State Virtual Machine Runtime
+  Copyright (c) 2023–2025 Robert A. James. All rights reserved.
+  Licensed under the StarForth License, Version 1.0.
+ */
+
+/**
+ * plic.h - Platform-Level Interrupt Controller interface (riscv64 only)
+ *
+ * Item 4.3.5a (FABRIC.md §27.5): Phase 0 (0.2/0.3) only ever enabled the
+ * S-mode *timer* interrupt (sie.STIE). External interrupts (sie.SEIE) were
+ * never touched, and the PLIC -- the only external-interrupt path on
+ * RISC-V, there is no legacy PIC or I/O APIC equivalent -- had no driver
+ * at all before this item. Pure substrate: this item wires the mechanism
+ * (claim/dispatch/complete) with no permanent source enabled by default;
+ * a real consumer (4.3.5b, virtio-keyboard) enables its own source later.
+ */
+
+#ifndef STARKERNEL_PLIC_H
+#define STARKERNEL_PLIC_H
+
+#include <stdint.h>
+
+/**
+ * Map the PLIC and set the S-mode hart-0 context's priority threshold to 0
+ * (maximally permissive -- safe because nothing is enabled at any source
+ * by default; enabling a source is what actually lets it reach claim()).
+ * Also sets sie.SEIE. Does not touch sstatus.SIE -- arch_enable_interrupts()
+ * still owns that, same as the timer.
+ *
+ * @return 0 on success.
+ */
+int plic_init(void);
+
+/** Set a source's interrupt priority (1-7; 0 means "never interrupt"). */
+void plic_set_priority(uint32_t irq, uint32_t priority);
+
+/** Enable a source for the S-mode hart-0 context. */
+void plic_enable(uint32_t irq);
+
+/** Disable a source for the S-mode hart-0 context. */
+void plic_disable(uint32_t irq);
+
+/**
+ * Claim the highest-priority pending interrupt for the S-mode hart-0
+ * context. Returns the source ID, or 0 if none is pending.
+ */
+uint32_t plic_claim(void);
+
+/** Signal completion of the source previously returned by plic_claim(). */
+void plic_complete(uint32_t irq);
+
+#endif /* STARKERNEL_PLIC_H */