kernel: tagged log ring — per-line pid/name/level records, klog_status

Replace the linear keep-earliest RAM buffer with a 512 KiB ring of
framed records (log-ring.zig, host-tested): every debug_write becomes
one record per payload line, stamped by the kernel with the sender's
pid, task name (its binary path), level, per-boot sequence number, and
monotonic timestamp. Attribution is structural — a payload cannot forge
another sender's tag, and newline injection lands inside the forger's
own next record. Oldest records are overwritten when full; sequence
gaps make the loss countable.

debug_write gains a level argument (err/warn/info/debug/raw; old
two-arg callers clamp to raw). klog_read becomes a stream-offset read
that fails once the cursor falls behind the ring's tail; the new
klog_status (#45) returns the cursors plus the boot wall-clock anchor —
what the logger service will name per-boot log directories with.

The log now guards itself with a dedicated spinlock (BKL -> log lock
order, never the reverse); panic paths use a bounded try-acquire and
fall back to sinks-only. Serial rendering keeps the historical
transcript byte-identical for kernel and legacy raw output; leveled
records get a kernel-rendered name prefix. log-flush/init's interim
drains start at the ring tail and write framed records until the logger
service replaces them.

build.zig

@@ -875,6 +875,7 @@ pub fn build(b: *std.Build) void {
     for ([_][]const u8{
         "system/boot-handoff.zig",
         "system/abi.zig",
+        "system/initial-ramdisk.zig", // v2 path-named entries: find/basename/magic
         "system/devices/device-abi.zig",
         "system/devices/pci-class.zig", // class/subclass/prog-IF name decoding
         "system/devices/acpi-ids.zig", // _HID name decoding
@@ -921,6 +922,21 @@ pub fn build(b: *std.Build) void {
     });
     test_step.dependOn(&b.addRunArtifact(xkb_tests).step);
 
+    // The tagged kernel log ring: append/wrap/reclaim/sequence-gap behavior over
+    // a RAM buffer. Needs the `abi` module (record header layout), so it doesn't
+    // fit the plain loop above.
+    const log_ring_tests = b.addTest(.{
+        .root_module = b.createModule(.{
+            .root_source_file = b.path("system/kernel/log-ring.zig"),
+            .target = target,
+            .optimize = optimize,
+            .imports = &.{
+                .{ .name = "abi", .module = abi_module },
+            },
+        }),
+    });
+    test_step.dependOn(&b.addRunArtifact(log_ring_tests).step);
+
     // runtime.time's Instant/Duration arithmetic. time.zig pulls in system.zig (the
     // syscall wrappers), which needs the `abi` module, so it doesn't fit the plain
     // loop above.