Establish buildable modules and shared host-service boundaries

designer-contracts/src/main/java/de/neitzel/fxdesigner/contracts/DocumentSnapshot.java

@@ -0,0 +1,14 @@
+package de.neitzel.fxdesigner.contracts;
+
+import java.net.URI;
+import java.util.Objects;
+
+/** Immutable host-owned draft; revision ordering is independent of native undo stamps. */
+public record DocumentSnapshot(
+    URI document, long revision, String text, boolean writable, boolean dirty) {
+  public DocumentSnapshot {
+    Objects.requireNonNull(document);
+    Objects.requireNonNull(text);
+    if (revision < 0) throw new IllegalArgumentException("Negative revision");
+  }
+}

designer-contracts/src/main/java/de/neitzel/fxdesigner/contracts/EditBatch.java

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+package de.neitzel.fxdesigner.contracts;
+
+import java.util.*;
+
+/** One named undo transaction. All UTF-16 ranges address the same expected revision. */
+public record EditBatch(long expectedRevision, String name, List<Replacement> replacements) {
+  public record Replacement(int start, int end, String text) {
+    public Replacement {
+      if (start < 0 || end < start) throw new IllegalArgumentException("Invalid range");
+      Objects.requireNonNull(text);
+    }
+  }
+
+  public EditBatch {
+    if (expectedRevision < 0) throw new IllegalArgumentException("Negative revision");
+    if (name == null || name.isBlank()) throw new IllegalArgumentException("Missing command name");
+    replacements = List.copyOf(replacements);
+  }
+
+  /** Validates every range before computing a result; never partially edits a host draft. */
+  public String applyTo(DocumentSnapshot draft) {
+    if (!draft.writable()) throw new IllegalStateException("Document is read-only");
+    if (draft.revision() != expectedRevision)
+      throw new ConcurrentModificationException("Stale document revision");
+    var ordered = new ArrayList<>(replacements);
+    ordered.sort(Comparator.comparingInt(Replacement::start));
+    int previousEnd = -1;
+    for (var r : ordered) {
+      if (r.end() > draft.text().length() || r.start() < previousEnd)
+        throw new IllegalArgumentException("Overlapping/out-of-bounds edits");
+      previousEnd = r.end();
+    }
+    var result = new StringBuilder(draft.text());
+    for (int i = ordered.size() - 1; i >= 0; i--) {
+      var r = ordered.get(i);
+      result.replace(r.start(), r.end(), r.text());
+    }
+    return result.toString();
+  }
+}