refactor(dos): centralize MZ admission and load-image evidence

src/domain/binaryTarget.ts

@@ -20,6 +20,7 @@ import { readElfFixedHeader } from "./elfFixedHeader.js";
 import {
   mzWindowsHeaderLayoutLimitation,
   parseMzHeaderEvidence,
+  type MzHeaderEvidence,
   type MzWindowsHeaderSignatureEvidence,
 } from "./dosMz.js";
 import { decodePeMachine, readPeOptionalHeader } from "./peMachine.js";
@@ -126,7 +127,10 @@ export const parseExecutableHeader = (
   )
     return parseElf(bytes, fileSize);
   if (bytes.length >= 2 && bytes[0] === 0x4d && bytes[1] === 0x5a)
-    return parseMz(bytes, fileSize, mzWindowsHeaderSignature);
+    return parseMzExecutableMetadata(
+      bytes,
+      parseMzHeaderEvidence(bytes, fileSize, mzWindowsHeaderSignature),
+    );
   if (bytes.length < 8) return err("truncated or unsupported binary header");
   const magic = bytes.readUInt32BE(0);
   if ([0xfeedface, 0xfeedfacf, 0xcefaedfe, 0xcffaedfe].includes(magic))
@@ -434,12 +438,11 @@ const parseElf = (
   });
 };
 
-const parseMz = (
+/** Select executable metadata from the reader's completed MZ observations. */
+export const parseMzExecutableMetadata = (
   bytes: Buffer,
-  fileSize: number,
-  signatureAtOffset?: MzWindowsHeaderSignatureEvidence,
+  evidence: MzHeaderEvidence,
 ): Result<ExecutableMetadata, string> => {
-  const evidence = parseMzHeaderEvidence(bytes, fileSize, signatureAtOffset);
   const { windowsHeader } = evidence;
   if (
     windowsHeader.state === "header-needed" ||