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

project.yml

@@ -6,6 +6,11 @@ options:
   xcodeVersion: "26.0"
   defaultConfig: Debug
 
+packages:
+  Yams:
+    url: https://github.com/jpsim/Yams.git
+    from: 6.0.0
+
 settings:
   base:
     SWIFT_VERSION: "6.0"
@@ -19,6 +24,8 @@ targets:
     platform: macOS
     sources:
       - Kanban
+    dependencies:
+      - package: Yams
     postBuildScripts:
       - script: '"${SRCROOT}/../indie-skills/skills/app-versioning/scripts/update_build_info.sh"'
         name: Update Build Info
@@ -42,6 +49,9 @@ targets:
     platform: macOS
     sources:
       - KanbanTests
+      - path: Fixtures
+        type: folder
+        buildPhase: resources
     dependencies:
       - target: Kanban
     settings: