feat(default)!: model default(...) as a real LCA-aware transition

`default(...)` is now a transition with the same semantics as any
event-driven transition. It fires immediately after the declaring
state's entries finish and its durings start, applying the standard
exit-to-LCA / enter-to-target algorithm. Three behavioral changes
follow from this unification:

* Optional. A state with children is no longer required to declare a
  `default(...)`. When omitted, the state itself is the resting
  innermost active state; substates are reachable only via explicit
  transitions. This is the "microwave Standby" pattern — Standby
  has substates like LowPower that you enter only on demand, and
  `=> Standby;` lands at Standby itself with no descent.

* Targets anywhere. The default target may be any state in the chart
  — a child (the classic descent case), a sibling, an ancestor,
  anywhere. `default(<sibling-or-ancestor>)` exits the just-entered
  state as part of the default-fire transition; the same rule fires
  whether you wrote it for descent, redirection, or up-bounce.

* Cycle-checked at compile time. The default-edge graph (out-degree
  ≤ 1 per node) is rejected if it contains any cycle, including
  `default(self)`. `state A { default(A); }` and `A → B → A` chains
  fail to compile with a pinpoint error message naming the cycle.
  This guarantees default chains terminate at runtime.

Implementation:

* `validate.rs`: drop the "must be direct child" and "must have
  children" restrictions; add `validate_default_graph` (iterative
  three-color cycle detection via chain walk).

* `ir.rs`: keep the unresolved default ident through to
  `resolve_transitions` so non-child targets resolve against the
  global state-name map (the same map transition targets use).

* `codegen.rs`: precomputed `(from, to)` paths no longer fold default
  descent into the enter slice — each path is LCA-relative
  `(exit, enter)` ending exactly at the explicit target.
  `descend_defaults` (a parent-pointer walk) is replaced by
  `follow_defaults`, which fires real transitions through
  `apply_path` and emits `TransitionFired{Internal}` /
  `TransitionComplete` brackets per link. Up-transitions don't
  trigger default-following (nothing was freshly entered, so no
  state's default fires).

* `active_durings_per_innermost_state`: predicate widened to include
  composites without a default (they can now be the innermost active
  state, so any durings they declare must be raced when current).

BREAKING CHANGES:

* Journal format. Default-fires now emit `TransitionFired` (with
  `reason: TransitionReason::Internal`) and `TransitionComplete`
  brackets around their entry sequence. `Internal` was previously
  reserved and never emitted; it is now active. Tests that
  hand-build expected journal sequences must include these brackets
  around every default-fire.

* Validation. `default(<non-child>)` and `default` on a childless
  state previously raised compile-time errors. Both are now valid.
  Code relying on these as guard rails must adapt.

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

hsmc-macros/tests/ui/t9_2_missing_default.rs

@@ -1,24 +0,0 @@
-use hsmc::statechart;
-
-#[derive(Debug, Clone)]
-pub enum Ev { _U }
-
-pub struct Ctx;
-
-statechart! {
-M {
-    context: Ctx;
-    events: Ev;
-    default(Parent);
-    state Parent {
-        state ChildA { entry: e; }
-        state ChildB { entry: e; }
-    }
-}
-}
-
-impl MActions for MActionContext<'_> {
-    fn e(&mut self) {}
-}
-
-fn main() {}

hsmc-macros/tests/ui/t9_2_missing_default.stderr

@@ -1,5 +0,0 @@
-error: state `Parent` has children but no `default` declaration
-  --> tests/ui/t9_2_missing_default.rs:13:11
-   |
-13 |     state Parent {
-   |           ^^^^^^

hsmc-macros/tests/ui/t9_4_default_non_child.rs

@@ -1,25 +0,0 @@
-use hsmc::statechart;
-
-#[derive(Debug, Clone)]
-pub enum Ev { _U }
-
-pub struct Ctx;
-
-statechart! {
-M {
-    context: Ctx;
-    events: Ev;
-    default(Parent);
-    state Parent {
-        default(Cousin);
-        state Child { entry: e; }
-    }
-    state Cousin { entry: e; }
-}
-}
-
-impl MActions for MActionContext<'_> {
-    fn e(&mut self) {}
-}
-
-fn main() {}

hsmc-macros/tests/ui/t9_4_default_non_child.stderr

@@ -1,5 +0,0 @@
-error: `default(Cousin)` in state `Parent`: `Cousin` is not a direct child of `Parent`
-  --> tests/ui/t9_4_default_non_child.rs:14:9
-   |
-14 |         default(Cousin);
-   |         ^^^^^^^

hsmc-macros/tests/ui/t9_4_default_self_cycle.rs

@@ -1,3 +1,6 @@
+// `default(self)` is a 1-cycle: entering A would fire its default
+// transition to A, which would fire its default again, ad infinitum.
+// Compile-time error.
 use hsmc::statechart;
 
 #[derive(Debug, Clone)]
@@ -9,9 +12,9 @@ statechart! {
 M {
     context: Ctx;
     events: Ev;
-    default(Leaf);
-    state Leaf {
-        default(Ghost);
+    default(A);
+    state A {
+        default(A);
         entry: e;
     }
 }

hsmc-macros/tests/ui/t9_4_default_self_cycle.stderr

@@ -0,0 +1,5 @@
+error: default-transition cycle detected: A -> A (entering any state in this cycle would loop forever before reaching user code)
+  --> tests/ui/t9_4_default_self_cycle.rs:17:9
+   |
+17 |         default(A);
+   |         ^^^^^^^

hsmc-macros/tests/ui/t9_5_default_childless.stderr

@@ -1,5 +0,0 @@
-error: `default` in state `Leaf` which has no child states
-  --> tests/ui/t9_5_default_childless.rs:14:9
-   |
-14 |         default(Ghost);
-   |         ^^^^^^^

hsmc-macros/tests/ui/t9_5_default_two_cycle.rs

@@ -0,0 +1,30 @@
+// A → B → A is a default cycle: entering either state would loop
+// forever before reaching user code. Compile-time error.
+use hsmc::statechart;
+
+#[derive(Debug, Clone)]
+pub enum Ev { _U }
+
+pub struct Ctx;
+
+statechart! {
+M {
+    context: Ctx;
+    events: Ev;
+    default(A);
+    state A {
+        default(B);
+        entry: e;
+    }
+    state B {
+        default(A);
+        entry: e;
+    }
+}
+}
+
+impl MActions for MActionContext<'_> {
+    fn e(&mut self) {}
+}
+
+fn main() {}

hsmc-macros/tests/ui/t9_5_default_two_cycle.stderr

@@ -0,0 +1,5 @@
+error: default-transition cycle detected: A -> B -> A (entering any state in this cycle would loop forever before reaching user code)
+  --> tests/ui/t9_5_default_two_cycle.rs:16:9
+   |
+16 |         default(B);
+   |         ^^^^^^^