docs(design): mark project RETIRED (paused) at the ledger spine

Owner decision, 2026-06-03: pause AILang. Status banner at the top of
design/INDEX.md (the sole addressable spine), so any reader — human or
agent — sees the state before the framing.

The differentiating thesis (a language purpose-built for LLM authorship
makes LLM authors measurably more correct) did not find support in the
project's own cross-model measurements
(experiments/2026-05-12-cross-model-authoring/format-findings.md):

- the optimal surface is model-relative — depth-annotated parens help a
  3B-active model (6/8 -> 7/8) but HURT a frontier model (7/8 -> 6/8,
  Llama-3.1-405B hallucinates structure it does not need);
- a compiler-in-the-loop is not an equaliser — agentic Qwen collapses
  into generation repetition before a single valid tree exists, so the
  oracle never engages;
- for a frontier author, AILang is at best neutral vs. a familiar
  language given a scan-plus-iterate workflow, not an edge.

The hallucination-robustness pillar (content-addressed symbols) was
separately falsified and removed (#66).

What survives, and why this is a pause not a deletion: the language is a
sound small ML/effect design, and the measurement harness is the most
durable output. main HEAD, git history, and the design ledger are
untouched; reversible if a new framing makes the remaining work
worthwhile.

design/INDEX.md

@@ -1,5 +1,21 @@
 # AILang Design — Index
 
+> **Status: RETIRED (paused) as of 2026-06-03.** No further compiler or
+> language work is planned for now. The project's differentiating thesis —
+> that a language purpose-built for LLM authorship makes LLM authors
+> measurably more correct — did not find empirical support in the project's
+> own cross-model measurements: the optimal surface is model-relative (a
+> redundant-structure crutch helps a small model and *hurts* a frontier one),
+> a compiler-in-the-loop is not an equaliser, and for a frontier author AILang
+> is at best neutral versus a familiar language given a scan. The
+> hallucination-robustness pillar (content-addressed symbols) was separately
+> falsified and removed (#66). Evidence corpus:
+> `experiments/2026-05-12-cross-model-authoring/format-findings.md`. The
+> language design below remains a sound small ML/effect language and the
+> measurement harness remains the project's most durable output; the git
+> history and the `design/` ledger stand as the record. This is a pause, not
+> a deletion — reversible if a new framing makes the remaining work worthwhile.
+
 The sole addressable entry point. Every contract and model is
 reached from here. A contract is a prescriptive, test-linked
 invariant; a model is a whitepaper narrative. `ratifying-test` names