iter prep.3-kernel-tier-modules (DONE 9/9): kernel-tier modules + param-in + stub crate — closes #33

Terminal iteration of the kernel-extension-mechanics milestone. Ships
the four language-level mechanisms named in the spec's § Goal:
Module.kernel + TypeDef.param-in schema, their Form-A surface,
flag-driven kernel-tier auto-injection, and generic param-in checker
enforcement with a new diagnostic.

Schema (Tasks 1+2). Module gains a `kernel: bool` field
(skip_serializing_if = is_false), TypeDef gains a
`param_in: BTreeMap<String, BTreeSet<String>>` field
(skip-if-empty, kebab-renamed to "param-in"). Both fields are
strictly additive — every pre-existing fixture's canonical-JSON hash
is bit-stable except `prelude.ail`, which intentionally gains
`(kernel)`. The struct-literal sweep covered ~104 Module sites and
~35 TypeDef sites across the workspace; the additive serde-default
covers JSON deserialise paths, only Rust struct literals broke.

Form-A surface (Tasks 3+4). `(kernel)` is a bare module-header
attribute; `(param-in (a Int Float) (b Str))` is one outer
TypeDef-body clause carrying one or more inner var-lists (OQ1
decision — mirrors `(ctors …)`, one parser arm, deterministic
BTreeMap iteration). Both round-trip Form-A → JSON → Form-A
bit-identical.

Workspace-load migration (Task 5). The hardcoded `&["prelude"]`
literal at loader.rs:108 became a `modules.values().filter(|m|
m.kernel)` derivation; `parse_prelude()` injection stays because
the prelude has no on-disk manifest in user workspaces. Prelude
now carries `(kernel)` in its source, so the new filter picks it
up automatically. Code-path migration only — observable behaviour
is identical (prelude_free_fns.rs stays green). prelude hash
re-pinned (af372f28c726f29f) with Honesty-Rule provenance comment.
WorkspaceLoadError::ReservedModuleName diagnostic prose
repurposed: any built-in kernel module name is reserved
(currently prelude + kernel_stub), not specifically prelude. CLI
mapping at main.rs updated in lockstep.

Stub crate (Task 6). New `crates/ailang-kernel-stub/` is a
zero-dependency leaf crate carrying only `pub const STUB_AIL:
&str` with the Form-A source of the kernel_stub module (one
parametric TypeDef with param-in, one ctor). The parse hop —
`parse_kernel_stub()` — lives in ailang-surface next to
parse_prelude, keeping the crate-dependency graph acyclic
(`ailang-surface → ailang-kernel-stub → ailang-core`, no
back-edge). The stub is injected unconditionally in all builds as
the ratifying fixture for the kernel-extension mechanism; future
base extensions may add more or retire the stub. Drift-pinned by
`kernel_stub_module_round_trips`.

Checker (Task 7). New `CheckError::ParamNotInRestrictedSet`
variant + code() + ctx() arms + enforcement in
`check_type_well_formed`'s Type::Con arm — generic, data-driven
from the TypeDef, mentions no specific extension type. Two
in-source tests pin both the rejection (`Str` outside `{Int,
Float}`) and the acceptance (`Int` inside) paths.

Workspace-load integration tests (Task 8). New
`workspace_kernel.rs` integration-test crate with three tests:
auto-import without explicit `(import …)` declaration, two
kernel-tier modules co-load, explicit-import-overrides-auto-
import precedence preserved. Loader is import-tree-only so the
auto-import tests use a bridge module that brings the kernel
module into the workspace via the import graph — docstring
captures the reachability nuance for future readers.

Doc-state transitions (Task 9). INDEX.md kernel-extensions row
annotation transitions from "design accepted 2026-05-28; impl in
progress" to "mechanisms milestone closed 2026-05-28; raw-buf and
series milestones pending". Whitepaper STATUS + auto-import +
param-in sections transitioned forward→present for shipped
mechanisms; forward-tense survives only in sections describing
the still-pending raw-buf/series milestones (per Honesty-Rule).
data-model contract gains anchor blocks for both new schema
fields.

Side-effect: every binary's IR snapshot now contains ~52 lines
for `drop_kernel_stub_StubT` because the stub is auto-injected
into every workspace load. Snapshots refreshed; e2e expects 4
modules per workspace (prelude + kernel_stub + entry + zero or
more user modules) instead of the previous 3.

Plan defects scrubbed in the implementation (folded back into
the planner template via the planner's self-review checklist
next time): Task 4 sample test src used fictional
`(ctors (MkT a))` list form (project grammar is per-`(ctor MkT
a)`); Task 6 original wiring would have created a cycle
ailang-surface → ailang-kernel-stub → ailang-surface (inverted —
stub crate is zero-dep, parse hop lives in surface); Task 7 in-
source tests referenced a fictional `check_type_in_module`
helper (used the existing Workspace + check_workspace
convention); Task 8 first integration test expected loader to
auto-load kernel modules from disk (loader is import-tree-only;
tests use a bridge module).

Concern-5 fix folded in pre-commit: workspace.rs ReservedModuleName
doc-prose initially said "in test/dev builds" for kernel_stub —
but stub is unconditionally injected in all builds. Doc copy
tightened to present-state per Honesty-Rule.

Stats: 0 spec-review-loops, 0 quality-review-loops, 2 sweep-script
retries on Task 2 (brace-depth bug on nested vec![Ctor{…}],
recovered via per-file checkout + rewritten anchor-on-existing-
field sweep), 1 e2e-snapshot refresh on Task 6.

crates/ail/src/main.rs

@@ -546,6 +546,7 @@ fn main() -> Result<()> {
                     let one = ailang_core::Module {
                         schema: m.schema.clone(),
                         name: m.name.clone(),
+                        kernel: false,
                         imports: vec![],
                         defs: vec![def.clone()],
                     };
@@ -569,6 +570,7 @@ fn main() -> Result<()> {
                     let one = ailang_core::Module {
                         schema: m.schema.clone(),
                         name: m.name.clone(),
+                        kernel: false,
                         imports: vec![],
                         defs: vec![def.clone()],
                     };
@@ -1229,13 +1231,17 @@ fn workspace_error_to_diagnostic(
                 "type": type_repr,
             })),
         ),
-        // the user named a module `prelude`, which the
-        // loader reserves for the auto-injected prelude module.
+        // the user named a module whose name collides with a
+        // built-in kernel-tier module that the loader auto-injects
+        // (e.g. `prelude`). Since prep.3 of the kernel-extension-
+        // mechanics milestone, the reserved set is derived from the
+        // kernel-tier filter; the diagnostic message generalises
+        // accordingly.
         W::ReservedModuleName { name } => Some(
             ailang_check::Diagnostic::error(
                 "reserved-module-name",
                 format!(
-                    "module name {name:?} is reserved (auto-injected by the loader)"
+                    "module `{name}` collides with a built-in kernel-tier module name (reserved)"
                 ),
             )
             .with_ctx(serde_json::json!({