file-system: extract the serving harness from fat — V1

fat was one binary doing four jobs; the three that are not FAT-specific move
to library/kernel/file-system-harness, a Server(comptime Engine) generic over
the engine type: the badge-scoped open-node table, the nine vfs handlers, the
not-mounted politeness, the exit sweep, mount registration, and durable-on-
close. A filesystem is now an engine plus a main that hands the harness a
mounted volume; a second engine reuses the harness wholesale.

Placement note: the plan said library/file-system, but the harness is a
specialization of `service` (its sibling) and needs nothing from the device
domain, so it lives beside service in library/kernel and stays block-free —
durability rides a caller closure (Volume.flush), no backwards kernel->device
dependency, no new-domain scaffolding. The engine type is inferred from
resolve()'s return, so engine.zig is untouched (its Node stays module-scope).

fat keeps only its FAT-specific bring-up (acquireVolume, DMA, engine.mount,
the attach/detach round trip) and the three mount prefixes as data. Behavior-
neutral: 13/13 across the fat/vfs/logger/IOMMU surface, nothing observable
changed. This lands first so every later phase touches the harness once.

system/services/fat/build.zig

@@ -10,10 +10,9 @@ pub fn build(b: *std.Build) void {
         .name = "fat",
         .root_source_file = b.path("fat.zig"),
         .imports = &.{
-            "block",                    "channel", "device-manager-protocol", "driver",
-            "envelope",                 "file-system", "ipc",                 "logging",
-            "memory",                   "process", "service",                 "time",
-            "vfs-protocol",
+            "block",     "channel",             "device-manager-protocol",
+            "driver",    "envelope",            "file-system-harness",
+            "ipc",       "logging",             "memory",
         },
     });
     b.installArtifact(exe);