feat(observe)!: unify journal and trace into single observation pipeline

The codegen now emits one `__chart_observe!` invocation per observable
atom. The macro fans out at compile time to whichever sinks are enabled:
the in-memory journal (`feature = "journal"`), `defmt` (`trace-defmt`),
`log` (`trace-log`), and/or `tracing` (`trace-tracing`). Trace and
journal share the same vocabulary by construction — they cannot
diverge. With no sink feature on, every site expands to `()` (zero
overhead). Multiple trace backends now compose; previously they were
mutually exclusive.

Trace lines use a logfmt-style format with a `[statechart:Name]` prefix
and `key=value` pairs, machine-parseable and greppable. Twenty verbs
cover every observable atom — entries/exits with begin/end pairs,
actions with kind, durings, transitions with the reason that triggered
them, event lifecycle (received/delivered/dropped/queued/failed),
timers, and termination.

BREAKING CHANGES (chart syntax unchanged; observability surface only):

- `TraceEvent::TransitionFired` now carries `reason: TransitionReason`
  showing whether the transition was driven by an event, a timer, or
  internal logic.
- New `TraceEvent` variants for phase pairing: `EnterBegan`, `ExitBegan`,
  `TransitionComplete`, `EventReceived`.
- Trace prefix changed from `[Name]` to `[statechart:Name]`.
- The chart-level `trace;` keyword is parsed for backwards compatibility
  but no longer gates emission.
- `__chart_trace!` / `__chart_journal!` macros removed (doc-hidden).

Verification: 159 integration tests pass, 0 missed mutants, miri 68
tests clean, Creusot 14/14 VCs discharged. Zero overhead confirmed via
`cargo expand` — every observation site expands to `{}` with no sinks
enabled.

hsmc-macros/src/ir.rs

@@ -26,10 +26,12 @@ pub struct Ir {
     /// Duration triggers referenced anywhere, in first-seen order.
     /// Each one gets a trigger id used by the timer table.
     pub duration_triggers: Vec<DurationTrigger>,
-    /// True when the chart declared `trace;` at root. Codegen routes
-    /// trace calls to `::hsmc::__chart_trace!`, which dispatches to a
-    /// backend (defmt / log / tracing / no-op) based on the user's
-    /// `hsmc/trace-*` feature selection in their `Cargo.toml`.
+    /// Set when the chart declared `trace;` at root. Retained for
+    /// backwards-compatible parsing — the keyword no longer gates
+    /// observation emission. As of 0.4 every chart unconditionally
+    /// emits `__chart_observe!` calls, and the actual trace fan-out is
+    /// chosen by the user's `hsmc/trace-*` cargo feature selection.
+    #[allow(dead_code)]
     pub trace_enabled: bool,
 }