iter cma.1: master mini-spec + render binary for cross-model authoring experiment

New top-level experiments/2026-05-12-cross-model-authoring/ (out of
root Cargo workspace; nested-crate standalone via empty [workspace]
table per Cargo idiom). The renderer projects one canonical
master/spec.md into two form-specific files (rendered/json.md and
rendered/ailx.md) by walking three directive forms ({form-only:
json}, {form-only: ailx}, {example: <id>}) and emitting per-example
fenced blocks sourced from 13 curated .ail.json fixtures.

Four test gates green (10/10 total):
- splitter_unit (7) — directive parser, malformed inputs panic
- spec_completeness (1) — AST-variant visitor lifted verbatim from
  schema_coverage.rs; 34/34 variants covered by the curated subset
- example_roundtrip (1) — every fixture roundtrips through
  ailang_surface::{print, parse} to identical canonical bytes
- token_balance (1) — form-only block totals balanced to 3.22% under
  tiktoken-rs::cl100k_base (gate is ±5%)

Six of the 34 variants did not have a dedicated fixture topic in the
plan and were absorbed into the closest existing fixture per the
plan's Step 4.14 escape hatch (Let/Clone → fn_calls_prelude; LetRec
→ data_with_match; If → match_literal_pattern; ReuseAs →
data_simple; Const → floats). Surfaced as a journal note, not a
plan revision.

Two ancillary additions vs. plan, both structural Cargo necessities,
spec-compatible: render/Cargo.toml carries an empty [workspace] table
so the nested crate refuses parent-workspace discovery, and
render/.gitignore drops /target + Cargo.lock. Both surfaced inline
in the per-iter journal's Concerns section.

cma.2 (harness) and cma.3 (live IONOS run + DESIGN.md addendum +
journal + roadmap edits) remain out of scope per the spec.

docs/journals/INDEX.md

@@ -20,3 +20,4 @@
 - 2026-05-12 — iter rt.2: DESIGN.md anchor for Roundtrip Invariant (top-level section + Decision 6 Constraint 2 upward cross-reference) → 2026-05-12-iter-rt.2.md
 - 2026-05-12 — audit-rt: milestone close (Roundtrip Invariant) — architect drift fixed in rt.tidy (3 items: Direction-2 5th test, roadmap P1 removed, wording sync); bench all-green → 2026-05-12-audit-rt.md
 - 2026-05-12 — iter boss: `/boss` skill — autonomous-mode discipline (Direction freedom, Notifications, WhatsNew procedure) extracted from CLAUDE.md into a user-invoked skill → 2026-05-12-iter-boss.md
+- 2026-05-12 — iter cma.1: master mini-spec + render binary for cross-model authoring experiment (13 fixtures cover 34/34 AST variants, 4 test gates green, rendered/{json,ailx}.md checked in; out-of-workspace nested crate) → 2026-05-12-iter-cma.1.md