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

Kanban/Storage/FrontmatterError.swift

@@ -0,0 +1,24 @@
+import Foundation
+
+/// Structural failures parsing an `index.md`. Everything here is fail-fast material
+/// (01-storage-format.md § Malformed input); lenient field problems never surface as errors.
+public enum FrontmatterError: Error, Equatable, Sendable, CustomStringConvertible {
+    case missingOpeningDelimiter
+    case missingClosingDelimiter
+    /// `line` is 1-based within the whole file, not within the YAML block.
+    case unparseableYAML(message: String, line: Int?)
+    case frontmatterNotAMapping
+
+    public var description: String {
+        switch self {
+        case .missingOpeningDelimiter:
+            "File does not start with a '---' frontmatter delimiter"
+        case .missingClosingDelimiter:
+            "Frontmatter is never closed by a '---' delimiter"
+        case let .unparseableYAML(message, line):
+            line.map { "Unparseable YAML at line \($0): \(message)" } ?? "Unparseable YAML: \(message)"
+        case .frontmatterNotAMapping:
+            "Frontmatter is not a YAML mapping"
+        }
+    }
+}