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/FrontmatterTests.swift

@@ -962,3 +962,37 @@ struct FrontmatterEmissionTests {
             == .mapping([YAMLValue.Pair(key: .string("order"), value: .int(7))]))
     }
 }
+
+// MARK: - Appended lines and empty-but-not-blank blocks
+
+struct FrontmatterAppendedLineTests {
+    @Test func appendedKeyAdoptsCRLFInACRLFFile() throws {
+        var document = try FrontmatterDocument.parse("---\r\nschema: 1\r\n---\r\nbody\r\n")
+        document.set("title", to: .string("x"))
+        #expect(document.serialized() == "---\r\nschema: 1\r\ntitle: x\r\n---\r\nbody\r\n")
+    }
+
+    @Test func appendedKeyStaysLFInAnLFFile() throws {
+        var document = try FrontmatterDocument.parse("---\nschema: 1\n---\nbody\n")
+        document.set("title", to: .string("x"))
+        #expect(document.serialized() == "---\nschema: 1\ntitle: x\n---\nbody\n")
+    }
+
+    /// `{}`, `null`, `~` resolve to an empty mapping yet leave text no key owns — appending
+    /// after it would be unparseable YAML, so the shape refuses edits (readable-but-uneditable).
+    @Test(arguments: ["---\n{}\n---\nbody\n", "---\nnull\n---\nbody\n", "---\n~\n---\nbody\n"])
+    func emptyButNotBlankFrontmatterIsUneditable(text: String) throws {
+        #expect(try FrontmatterDocument.parse(text).uneditableShape == .keyWithoutOwnLine)
+    }
+
+    /// Truly blank or comment-only frontmatter stays editable — appending after it is exactly
+    /// what `set` is for, and the result must reparse.
+    @Test(arguments: ["---\n---\nbody\n", "---\n\n---\nbody\n", "---\n# just a comment\n---\nbody\n"])
+    func blankOrCommentOnlyFrontmatterStaysEditable(text: String) throws {
+        var document = try FrontmatterDocument.parse(text)
+        #expect(document.uneditableShape == nil)
+        document.set("schema", to: .int(1))
+        let reparsed = try FrontmatterDocument.parse(document.serialized())
+        #expect(reparsed.schema == .valid(1))
+    }
+}