fix(delta-upgrade): normalize extractSha256 output to lowercase

Bun.CryptoHasher.digest('hex') always returns lowercase hex, but
SHA256_DIGEST_PATTERN with the /i flag could capture uppercase from
GitHub asset digests. Normalize to lowercase to ensure comparison
always succeeds regardless of upstream casing.

Author: BYK

src/lib/delta-upgrade.ts

@@ -150,7 +150,8 @@ export function extractSha256(asset: GitHubAsset): string | null {
     return null;
   }
   const match = SHA256_DIGEST_PATTERN.exec(asset.digest);
-  return match ? (match[1] ?? null) : null;
+  // Normalize to lowercase — Bun.CryptoHasher.digest("hex") returns lowercase
+  return match ? (match[1]?.toLowerCase() ?? null) : null;
 }
 
 /**

test/lib/delta-upgrade.test.ts

@@ -151,9 +151,9 @@ describe("extractSha256", () => {
     expect(extractSha256(asset)).toBeNull();
   });
 
-  test("handles uppercase hex", () => {
+  test("normalizes uppercase hex to lowercase", () => {
     const asset = makeAsset({ digest: "sha256:ABCDEF0123456789" });
-    expect(extractSha256(asset)).toBe("ABCDEF0123456789");
+    expect(extractSha256(asset)).toBe("abcdef0123456789");
   });
 
   test("handles mixed case prefix", () => {