Build the frontmatter engine with byte-perfect round-trip

Yams validates and reads; writes are surgical line-span edits over
retained raw text, so untouched bytes — unknown keys, comments, odd
formatting, bodies — round-trip identically by construction. Strict
schema/order readers distinguish valid/missing/malformed for the
loader's fail-fast; lenient fields preserve malformed values verbatim;
modified-by is schema-owned. Adds the Yams package and the fixture
folder-reference wiring. 44 tests (105 cases).

Claude-Session: https://claude.ai/code/session_018BjQRYBR6jQja3jCRi5S3A

Fixtures/README.md

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+# Fixture boards
+
+Golden fixture boards for the storage-contract test suite — real on-disk folder trees, not inline strings, so the same fixtures can later drive XCUITests via the `--open-board` launch hook.
+
+Bundled into the unit-test target as a folder reference (see `project.yml`). Valid boards live under `Valid/`, fail-fast cases under `Malformed/`.