iter prep.1-type-scoped-namespacing (DONE 5/5): TypeDef-first resolution + workspace pre-pass — closes #31

First iteration of the kernel-extension-mechanics milestone. Ships
the type-scoped `<TypeName>.<member>` resolution path as the
canonical form for type-associated operations, narrows the
`BareCrossModuleTypeRef` / `BadCrossModuleTypeRef` diagnostics from
"bare = strictly local" to "bare = in-scope by any path", migrates
12 std-library example fixtures, and introduces a workspace-wide
normalisation pre-pass `prepare_workspace_for_check` shared between
`check_workspace` and `monomorphise_workspace`.

Architectural discovery during implementation: the plan covered the
`Term::Var` dot-qualified resolver layer plus the workspace
validator's bare-name acceptance, but the migration of bare-form
fixtures exposed five sites where bare vs. qualified type-names
needed symmetric treatment — `Term::Ctor` resolution, `Type::Con`
well-formedness, mono's poly-free-fn name/constraint-count
enumeration, codegen's `lookup_ctor_by_type` bare-name path, and
the upstream desugar-then-qualify composition. Rather than
scattering TypeDef-first ladders across each site, the implementer
centralised the work into one pre-pass that walks every consumer
module's `Type::Con.name` and `Term::Ctor.type_name`, rewriting
bare cross-module references to their qualified `<home>.<Type>`
form. This is symmetric to the pre-existing `qualify_local_types`
(owner-side); the new pre-pass is the consumer-side mirror.
Downstream passes see qualified Types regardless of authoring form.
The TypeDef-first ladder still lives in `synth`'s `Term::Var` arm
because `<TypeName>.<member>` is term-position-only — `Maybe.from_maybe`
is a Var, not a Type expression, and the pre-pass does not rewrite
Var names.

Alternatives considered:

(a) Add TypeDef-first ladder at every resolution site separately
    (the plan's implicit assumption). Rejected: O(N) extension
    sites, each carrying the same workspace-walking logic; the
    pre-pass version is O(1) — one pass, every downstream consumer
    benefits.
(b) BLOCKED + spec re-brainstorm. Rejected: the architecture
    extension is consistent with prep.1's thesis (bare type-name
    resolves to the workspace-wide TypeDef) and forward-compatible
    with prep.2 (Term::New.type_name falls under the same rewrite)
    and prep.3 (kernel-tier TypeDefs enter the workspace map
    automatically). No design regression to bounce back over.

Spec updated to document the realisation mechanism honestly: the
"Realisation mechanism — workspace pre-pass" subsection clarifies
that the resolver-level semantics described in "Implementation
shape" are the user-facing contract, and the actual code path is
the pre-pass.

Verification:

- `cargo test --workspace`: ALL GREEN. 87 e2e + every crate's unit
  + integration tests pass with no regressions.
- Three NEW in-source tests pin Task 1's resolver paths:
  `type_scoped_member_resolves`, `type_scoped_member_not_found`,
  `type_scoped_receiver_not_a_type`.
- One NEW workspace test pins the narrowed validator:
  `ct1_validator_accepts_bare_with_explicit_import`.
- One renamed-and-flipped existing test:
  `ct1_validator_rejects_bare_xmod_with_import_candidate` →
  `ct1_validator_accepts_bare_xmod_with_import_candidate` (the
  bare-with-import path is now ACCEPTED).
- One NEW companion test for the workspace-wide ctor lookup:
  `ct2_term_ctor_bare_cross_module_via_workspace_resolves`.
- Two pre-existing tests' assertions updated for the new error
  wording: `ct1_check_cli::check_human_mode_emits_actionable_message_to_stderr`
  and `crates/ailang-check/tests/workspace.rs::unknown_module_prefix_is_reported`.
- 12 migrated `.ail` fixtures verified via the existing e2e
  suite (each fixture is the test runner's target for an existing
  `build_and_run` assertion).
- Negative fixture `ct_2_bare_cross_module.ail` semantically
  preserved: dropped its `(import std_maybe)` so bare `Maybe` is
  out-of-scope under the narrowed rule and still fires
  `BareCrossModuleTypeRef`.

Concerns:

- The pre-pass introduces a new architectural layer (consumer-side
  qualification) that the spec did not originally anticipate. Spec
  amendment in this commit documents the layer. Future iterations
  reference `prepare_workspace_for_check` as established
  infrastructure.
- `examples/test_ct1_bare_xmod_rejected.ail.json` switched its
  offending name from bare `Ordering` (which under the prep.1
  semantics may now resolve via implicit prelude) to a still-
  unresolvable `Mystery_Type`. The CLI test's intent (assert that
  a human-mode `ail check` exits non-zero on a still-RED case) is
  preserved.

Milestone status: kernel-extension-mechanics (Gitea #6) advances
1/3 iters. Next: prep.2 (`Term::New` construct) issue #32.

crates/ail/src/main.rs

@@ -1250,9 +1250,9 @@ fn workspace_error_to_diagnostic(
             ailang_check::Diagnostic::error(
                 "bare-cross-module-type-ref",
                 format!(
-                    "module `{module}` contains bare type name `{name}` that does not resolve to a local type; \
-                    candidates from imports: {candidates:?}. \
-                    Run `ail migrate-canonical-types` to fix legacy fixtures."
+                    "module `{module}` references bare type `{name}`, which is not in scope. \
+                    Add `(import <module>)` to bring it into scope (candidates: {candidates:?}), \
+                    or rewrite the call to type-scoped form `<TypeName>.<member>`."
                 ),
             )
             .with_ctx(serde_json::json!({
@@ -1267,8 +1267,9 @@ fn workspace_error_to_diagnostic(
             ailang_check::Diagnostic::error(
                 "bad-cross-module-type-ref",
                 format!(
-                    "module `{module}` references qualified type `{name}` but the owner module is not known \
-                    or does not declare a type by that name"
+                    "module `{module}` references qualified type `{name}` but the owner module \
+                    is not known in the workspace, or it declares no type by that name. \
+                    Use the bare type-name from an imported module instead."
                 ),
             )
             .with_ctx(serde_json::json!({