Build BoardWriter — atomic, minimal-touch writes

The single point through which every mutation becomes a filesystem
operation: read fresh from disk (strict byte-faithful UTF-8), refuse
readable-but-uneditable frontmatter shapes, apply the edit through the
span engine, stamp modified / clear modified-by, then temp-file+rename
atomically. Renumber-visible-children is the sole minimal-touch
exception, tombstones untouched. Structured BoardWriteError carries
operation + path + reason for the future banner surface.

Alongside, three edges the card surfaced:
- The loader now decodes byte-faithfully too, so a BOM'd or non-UTF-8
  index.md is rejected at load per the encoding contract, instead of
  loading via NSString's silent BOM strip and then refusing every write.
- An appended frontmatter line adopts the file's prevailing line ending
  (a CRLF file stays uniformly CRLF when a stamp first lands in it).
- Empty-but-not-blank frontmatter ({}, null, ~) is detected as
  uneditable — appending after it would be unparseable YAML.

30 new unit tests; 176 total green.

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

KanbanTests/BoardLoaderTests.swift

@@ -502,3 +502,43 @@ struct BoardLoaderFailFastTests {
         }
     }
 }
+
+// MARK: - Encoding strictness
+
+/// The loader decodes byte-faithfully (no NSString BOM-stripping) so the settled encoding
+/// contract (01-storage-format.md § Fractal layout ▸ Rules) actually holds at load time: a
+/// BOM'd file fails the frontmatter delimiter, a non-UTF-8 file is named as such — the same
+/// strict decode `BoardWriter` uses, so a file can never load here and then refuse every write.
+struct BoardLoaderEncodingTests {
+    @Test func bomPrefixedBoardIndexIsRejected() throws {
+        let fixture = try BoardFixture()
+        defer { fixture.tearDown() }
+
+        var bytes = Data([0xEF, 0xBB, 0xBF])
+        bytes.append(Data("---\nschema: 1\n---\nbody\n".utf8))
+        try bytes.write(to: fixture.root.appendingPathComponent("index.md"))
+
+        #expect { try BoardLoader.load(boardRoot: fixture.root) } throws: { error in
+            guard let loadError = error as? BoardLoadError,
+                  case .unparseableYAML = loadError.reason else { return false }
+            return loadError.path == "index.md"
+        }
+    }
+
+    @Test func nonUTF8BoardIndexIsRejected() throws {
+        let fixture = try BoardFixture()
+        defer { fixture.tearDown() }
+
+        // "café" in ISO-8859-1 — 0xE9 is not valid UTF-8.
+        var bytes = Data("---\nschema: 1\ntitle: caf".utf8)
+        bytes.append(0xE9)
+        bytes.append(Data("\n---\n".utf8))
+        try bytes.write(to: fixture.root.appendingPathComponent("index.md"))
+
+        #expect { try BoardLoader.load(boardRoot: fixture.root) } throws: { error in
+            guard let loadError = error as? BoardLoadError,
+                  case let .unparseableYAML(message, _) = loadError.reason else { return false }
+            return message == "file is not UTF-8"
+        }
+    }
+}