G.2 (v2.0.0): unified rng_get_bytes() entropy entry point; virtio-rng sole backend

The QEMU-verifiable slice of the real-hardware RNG driver (per FABRIC-3.md
§G.2). New include/starkernel/rng.h + src/starkernel/rng/rng.c provide the
single entropy entry point: rng_init() probes the backend set (v2.0.0:
virtio-rng only) and, on no backend, prints a loud boot-time warning while
rng_get_bytes() returns RNG_ERR_NO_BACKEND - never silently degrading to a
deterministic seed. The backend-selection switch in rng.c is the exact seam
v2.5.0's per-arch drivers (amd64 RDRAND, riscv64 Zkr, aarch64 peripheral) plug
into without touching the call path.

Consumers route through the unified layer instead of virtio-rng directly:
capsule_mint.c (identity seed + drive_uuid) and kernel_main.c phase 8
(rng_init()). virtio_rng.c stays as the sole backend. Built clean on
amd64/aarch64/riscv64. QEMU amd64 boot: POST 1012/0/0 + ok>, "rng: backend =
virtio-rng" + "entropy: ready", Zuse identity confirmed from thumbdrive -
mint/cert behavior unchanged.

FABRIC-3.md §G.2 v2.0.0 slice marked BUILT+VERIFIED.

src/starkernel/rng/rng.c

@@ -0,0 +1,60 @@
+/*
+  StarKernel — Unified entropy layer (rng_get_bytes)
+
+  Single entropy entry point for the kernel; see include/starkernel/rng.h for
+  the contract and the probe-and-refuse-loudly discipline this implements.
+
+  v2.0.0: virtio-rng is the sole active backend (QEMU-only, uniform across all
+  three arches). The backend-selection switch in rng_get_bytes() is the exact
+  seam where the v2.5.0 real per-arch drivers (amd64 RDRAND, riscv64 Zkr,
+  aarch64 peripheral RNG) plug in without touching the call path.
+*/
+
+#ifndef __STARKERNEL__
+#error "rng.c is kernel-only"
+#endif
+
+#include "starkernel/rng.h"
+#include "starkernel/virtio_rng.h"
+#include "starkernel/console.h"
+
+/* Number of entropy backends known to this build. Each maps to one case in
+ * rng_init() and rng_get_bytes(). v2.0.0 has exactly one: virtio-rng. */
+enum {
+    RNG_BACKEND_NONE = 0,
+    RNG_BACKEND_VIRTIO,   /* virtio-rng (QEMU, all three arches) */
+};
+
+static int g_rng_backend = RNG_BACKEND_NONE;
+
+int rng_init(void) {
+    /* Probe backends in priority order; first success wins. virtio-rng is
+     * the sole backend at v2.0.0; v2.5.0 adds real per-arch drivers here. */
+    if (virtio_rng_init() == 0) {
+        g_rng_backend = RNG_BACKEND_VIRTIO;
+        console_println("rng: backend = virtio-rng");
+        return 0;
+    }
+
+    /* Refuse loudly: never fall through to a deterministic seed. */
+    g_rng_backend = RNG_BACKEND_NONE;
+    console_println(
+        "rng: WARNING — no entropy backend available; rng_get_bytes() "
+        "will refuse (no deterministic seed fallback)");
+    return -1;
+}
+
+int rng_ready(void) {
+    return g_rng_backend != RNG_BACKEND_NONE;
+}
+
+int rng_get_bytes(uint8_t *buf, size_t n) {
+    switch (g_rng_backend) {
+    case RNG_BACKEND_VIRTIO:
+        return virtio_rng_get_bytes(buf, n);
+    default:
+        /* v2.5.0 real per-arch cases land here. No backend: refuse loudly,
+         * never return a deterministic throwaway. */
+        return RNG_ERR_NO_BACKEND;
+    }
+}