Iter 6: deps hardening, multi-diagnose, DESIGN audit

- ail deps now filters builtins, fn params, and let/match-pattern
  bindings. New value_names() in ailang-check::builtins is the single
  source of truth shared with the typechecker install path. walk_term
  threads a scope set; qualified `prefix.def` refs pass through.
- check_in_workspace returns Vec<CheckError>; check_workspace
  accumulates body diagnostics across defs and modules. Pass-1
  (top-level symbol table) and per-module type-def setup stay
  fail-fast — corrupt env would taint later diagnostics.
- DESIGN.md "What the MVP is NOT" was lying (ADTs, strings landed
  in Iter 2/3). Renamed to "What is not (yet) supported" and split
  into "not yet" + supported/smoke-tested. JOURNAL Iter 6 entry
  records the architecture self-check.

Tests: 47 green (was 44). +2 deps filter tests in e2e,
+1 multi-diagnose test in ailang-check workspace integration.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/DESIGN.md

@@ -227,14 +227,32 @@ ail build <module>               — full pipeline → binary
    hash.
 5. **CI pin** of the outputs in `tests/expected/`.
 
-## What the MVP is NOT
+## What is not (yet) supported
 
-- No ADTs / pattern matching (Phase 2).
-- No closures / higher-order functions (Phase 2).
-- No effect handlers (Phase 3).
-- No refinements / SMT (Phase 4).
-- No module system beyond imports (Phase 2).
-- No first-class strings. Only ints + bools + unit.
+Snapshot of the boundary at the end of Iter 6. Items move out of this list
+as iterations land; the JOURNAL records the exact iteration.
 
-The MVP is successful when `examples/sum.ail.json` produces a binary that
-prints the sum 1..10 = 55.
+- No closures / higher-order functions. Will require a typed IR stage (TIR)
+  before lowering.
+- No effect handlers — only the built-in IO and Diverge ops.
+- No refinements / SMT escalation.
+- No cross-module ADTs. ADTs are local to a module; ctor names must be
+  unique within their module but may collide across modules.
+- No visibility rules in imports. Every top-level def of an imported module
+  is reachable; there is no `pub` / `priv`.
+- No GC. The `Ctor` heap layout leaks. Acceptable for current example
+  programs; required before any longer-running program.
+
+What **is** supported (and used as the smoke test for the pipeline):
+
+- Int, Bool, Unit, **Str** as primitive types.
+- `if`, `let`, function calls, recursion.
+- Effects on function signatures, with `do op(args)` for direct effect
+  ops (`io/print_int`, `io/print_bool`, `io/print_str`).
+- **ADTs + flat pattern matching** (Iter 3). Sub-patterns of a Ctor
+  pattern are restricted to `Var` / `Wild`.
+- **Imports + qualified cross-module references** via dotted names
+  (Iter 5).
+
+Pipeline regression smoke test: `examples/sum.ail.json` produces a
+binary that prints 55.