test(utils): add unit tests for domain computation utilities (#299)

## Summary

Add unit tests for `domain.zig` — the domain computation utilities
used in BLS signature verification.

### Motivation
`domain.zig` had 0 test coverage despite being critical for signature
verification correctness. These functions compute the BLS signing domain
per the consensus spec.

🤖 Generated with AI assistance

---------

Co-authored-by: lodekeeper-z <lodekeeper-z@users.noreply.github.com>
Co-authored-by: bing <spiralladder@fastmail.com>

Author: lodekeeper-z

src/state_transition/utils/domain.zig

@@ -20,7 +20,7 @@ pub fn computeDomain(domain_type: DomainType, fork_version: Version, genesis_val
 
 /// Return the ForkVersion at an epoch from a Fork type
 pub fn forkVersion(fork: Fork, epoch: Epoch) Version {
-    return if (epoch < fork.epoch) fork.previousVersion else fork.currentVersion;
+    return if (epoch < fork.epoch) fork.previous_version else fork.current_version;
 }
 
 /// Used primarily in signature domains to avoid collisions across forks/chains.
@@ -31,3 +31,71 @@ pub fn computeForkDataRoot(current_version: Version, genesis_validators_root: Ro
     };
     try types.phase0.ForkData.hashTreeRoot(&fork_data, out);
 }
+
+const testing = std.testing;
+const constants = @import("constants");
+const DOMAIN_BEACON_PROPOSER = constants.DOMAIN_BEACON_PROPOSER;
+const DOMAIN_BEACON_ATTESTER = constants.DOMAIN_BEACON_ATTESTER;
+const DOMAIN_VOLUNTARY_EXIT = constants.DOMAIN_VOLUNTARY_EXIT;
+
+test "computeDomain - domain type is first 4 bytes" {
+    const fork_version = [4]u8{ 0x01, 0x00, 0x00, 0x00 };
+    const genesis_root = [_]u8{0} ** 32;
+    var domain: Domain = undefined;
+
+    try computeDomain(DOMAIN_VOLUNTARY_EXIT, fork_version, genesis_root, &domain);
+
+    try testing.expectEqualSlices(u8, &DOMAIN_VOLUNTARY_EXIT, domain[0..4]);
+}
+
+test "computeDomain - different domain types produce different domains" {
+    const fork_version = [4]u8{ 0x01, 0x00, 0x00, 0x00 };
+    const genesis_root = [_]u8{0xAA} ** 32;
+
+    var domain_a: Domain = undefined;
+    var domain_b: Domain = undefined;
+
+    try computeDomain(DOMAIN_BEACON_PROPOSER, fork_version, genesis_root, &domain_a);
+    try computeDomain(DOMAIN_BEACON_ATTESTER, fork_version, genesis_root, &domain_b);
+
+    try testing.expect(!std.mem.eql(u8, &domain_a, &domain_b));
+}
+
+test "computeDomain - different fork versions produce different domains" {
+    const genesis_root = [_]u8{0xBB} ** 32;
+
+    var domain_a: Domain = undefined;
+    var domain_b: Domain = undefined;
+    const version_a = [4]u8{ 0x01, 0x00, 0x00, 0x00 };
+    const version_b = [4]u8{ 0x02, 0x00, 0x00, 0x00 };
+
+    try computeDomain(DOMAIN_BEACON_PROPOSER, version_a, genesis_root, &domain_a);
+    try computeDomain(DOMAIN_BEACON_PROPOSER, version_b, genesis_root, &domain_b);
+
+    // First 4 bytes (domain type) are the same
+    try testing.expectEqualSlices(u8, domain_a[0..4], domain_b[0..4]);
+    // But the fork data root portion differs
+    try testing.expect(!std.mem.eql(u8, domain_a[4..32], domain_b[4..32]));
+}
+
+test "forkVersion - given epoch returns correct version" {
+    const fork: Fork = .{
+        .previous_version = [4]u8{ 0x01, 0x00, 0x00, 0x00 },
+        .current_version = [4]u8{ 0x02, 0x00, 0x00, 0x00 },
+        .epoch = 100,
+    };
+    try testing.expectEqualSlices(u8, &fork.previous_version, &forkVersion(fork, 99));
+    try testing.expectEqualSlices(u8, &fork.current_version, &forkVersion(fork, 100));
+    try testing.expectEqualSlices(u8, &fork.current_version, &forkVersion(fork, 200));
+}
+
+test "computeForkDataRoot - different inputs produce different roots" {
+    const genesis_root = [_]u8{0xDD} ** 32;
+    var root_a: Root = undefined;
+    var root_b: Root = undefined;
+
+    try computeForkDataRoot([4]u8{ 0x01, 0x00, 0x00, 0x00 }, genesis_root, &root_a);
+    try computeForkDataRoot([4]u8{ 0x02, 0x00, 0x00, 0x00 }, genesis_root, &root_b);
+
+    try testing.expect(!std.mem.eql(u8, &root_a, &root_b));
+}