volume-manager: content signature + the filesystems.csv map (S2)

partition.Volume gains a FilesystemKind signature (today .fat for every probed
volume; S4 adds a real VBR recognizer for exFAT) — the seam filesystems.csv keys
on to choose a service binary. New filesystem-map.zig parses
`/system/configuration/filesystems.csv` (signature, binary) into rules and
match()es a signature to its binary, mirroring the device registry: a signature
no row matches goes unserved, never guessed; slices point into the source
buffer. Three host tests (fat->binary, the binary is data-driven not hardcoded,
malformed rows counted); the test rule buffer is a named fixture size so the
bounds gate stays quiet. The VM's build gains the csv dependency and wires the
filesystem-map test into its package test step. Not yet consumed by the binary —
that lands when the VM loads the tables (step 4).

system/services/volume-manager/build.zig

@@ -17,14 +17,27 @@ pub fn build(b: *std.Build) void {
     });
     b.installArtifact(exe);
 
-    // Standalone `zig build test` for the partition parser; the root build keeps
-    // its aggregate test step.
-    const test_step = b.step("test", "Run the partition-parser unit tests");
-    const tests = b.addTest(.{
+    // Standalone `zig build test` for the parser + mount-map modules; the root
+    // build keeps its aggregate test step.
+    const csv = b.dependency("csv", .{});
+    const test_step = b.step("test", "Run the partition parser + mount-map unit tests");
+
+    const partition_tests = b.addTest(.{
         .root_module = b.createModule(.{
             .root_source_file = b.path("partition.zig"),
             .target = b.resolveTargetQuery(.{}),
         }),
     });
-    test_step.dependOn(&b.addRunArtifact(tests).step);
+    test_step.dependOn(&b.addRunArtifact(partition_tests).step);
+
+    // filesystem-map imports csv (and, by path, partition.zig), so its test
+    // module needs csv wired.
+    const filesystem_map_tests = b.addTest(.{
+        .root_module = b.createModule(.{
+            .root_source_file = b.path("filesystem-map.zig"),
+            .target = b.resolveTargetQuery(.{}),
+            .imports = &.{.{ .name = "csv", .module = csv.module("csv") }},
+        }),
+    });
+    test_step.dependOn(&b.addRunArtifact(filesystem_map_tests).step);
 }