feat(artifacts): inventory flat installer packages

A .pkg inventory recorded only the root hash. Flat installer packages
are xar archives, and their install scripts and payloads are what an
analyst needs to review.

A pure TypeScript xar reader, which also runs off macOS:
- reads the TOC (zlib XML) and verifies the TOC checksum;
- walks nested product-package members, such as Component.pkg/;
- decodes raw and zlib ("x-gzip") members, verifying each extracted
  SHA-1/MD5 checksum as the bytes stream;
- rejects unsafe member names.

Gzip-cpio Scripts and Payload members are expanded as nested containers
by a streaming odc/newc cpio reader. It reads members once, in archive
order, and holds at most one chunk. pbzx (xz) payloads are recorded
with a limitation that names pkgutil --expand-full. Unsupported member
encodings, traversal, and truncation fail with typed reasons.

@xmldom/xmldom (already installed through plist) becomes a direct,
pinned dependency for TOC parsing.

verify:installer-package builds component, product, and pbzx packages
with pkgbuild and productbuild. It checks the expanded Payload against
`pkgutil --payload-files`, script and payload content digests, product
nesting, and CLI/MCP parity, and runs in the macOS CI job.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -303,6 +303,7 @@ jobs:
       - run: npm run verify:keyed-archive
       - run: npm run verify:asset-catalog
       - run: npm run verify:macos-bundle
+      - run: npm run verify:installer-package
       - name: Native compiled NIB hierarchy regressions
         run: npm run test:focused -- tests/boundary/filesystem/nibNativeHierarchy.test.ts tests/boundary/filesystem/interfaceBuilderBundle.test.ts
       - name: Real binary metadata and Interface Builder integration