docs: add Contract sections + tag tests to contract items (Tier 2 — 10 files)

org/core-communication.org

@@ -29,6 +29,16 @@ The length prefix solves all three problems. The reader reads exactly 6 characte
 
 The 6-character hex length supports messages up to ~16MB (0xFFFFFF bytes). This is sufficient for any single message the agent would produce. Larger payloads should be split across multiple messages.
 
+** Contract
+
+1. (frame-message msg): serializes a plist message to a length-prefixed
+   string. The first 6 characters are the hex-encoded payload length.
+2. (read-framed-message stream): reads a framed message from a stream,
+   returning the deserialized plist. Consumes exactly the length-prefixed
+   bytes.
+3. Round-trip invariant: ~(read-framed-message (make-string-input-stream
+   (frame-message msg)))~ equals ~msg~.
+
 * Implementation
 
 ** Package Context
@@ -261,19 +271,20 @@ Verifies that the framing protocol correctly serializes and deserializes message
 (in-suite communication-protocol-suite)
 
 (test test-framing
+  "Contract 1: frame-message produces correct hex length prefix."
   (let* ((msg '(:type :EVENT :payload (:action :handshake)))
          (framed (frame-message msg)))
     (is (string= "00002C" (string-upcase (subseq framed 0 6))))))
 
 (test test-framing-round-trip
-  "A message should survive frame → read-frame without loss."
+  "Contract 3: frame → read-frame preserves message identity."
   (let* ((msg '(:type :EVENT :payload (:action :handshake :version "1.0") :meta (:source :tui)))
          (framed (frame-message msg))
          (unframed (read-framed-message (make-string-input-stream framed))))
     (is (equal msg unframed))))
 
 (test test-framing-empty-message
-  "An empty or simple message should still frame correctly."
+  "Contract 1: simple messages frame with valid hex length."
   (let* ((msg '(:type :ping))
          (framed (frame-message msg)))
     (is (> (length framed) 5))