fat: long-file-name creation, fast cluster allocation, makePath

createFile/createDirectory now build long-name chains: a mangled STEM~N
8.3 alias (collision-checked per directory), the standard rotate-add
checksum, and 13-UCS-2-per-entry pieces written last-logical-first into
a contiguous free-slot run (found sector-wise, growing the directory as
today). Write order is chain first, 8.3 entry last, so an interrupted
create leaves only skippable orphans. Uppercase-compliant 8.3 names
keep the bare-entry fast path; lowercase names now get a chain so their
exact case survives — matching tools/make-fat-image.py. rename stays
8.3-only (documented; nothing needs more yet).

allocateCluster drops its from-cluster-2 rescan (measured ~1 s/cluster
on a part-full volume, a 37 s shutdown flush in the lost M19 build):
a next-free hint (rewound by frees) plus a FAT-sector LBA cache turn
the scan into one device read per FAT sector.

mkdir now refuses an existing name (no duplicate entries), and
runtime.fs gains makePath (mkdir -p) for the logger's nested per-boot
directories. Engine tests cover the log-directory shape, ~N collisions,
chain unlink/reuse, the 8.3 fast path, and the checksum.

system/services/fat/fat.zig

@@ -203,7 +203,9 @@ fn onMessage(message: []const u8, out: []u8, sender: u32, capability: ?runtime.i
             return writeReply(out, .{ .status = 0 }, &.{});
         },
         .mkdir => {
-            const split = splitParent(payload[0..@min(payload.len, request.len)]);
+            const path = payload[0..@min(payload.len, request.len)];
+            if (filesystem.resolve(path) != null) return fail(out); // already exists — no duplicate entries
+            const split = splitParent(path);
             const parent = filesystem.resolve(split.parent) orelse return fail(out);
             if (filesystem.createDirectory(parent, split.leaf) == null) return fail(out);
             return writeReply(out, .{ .status = 0 }, &.{});