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/tests/ui/t9_8_unknown_target.rs

@@ -1,3 +1,5 @@
+#![allow(unexpected_cfgs)]
+
 use hsmc::statechart;
 
 #[derive(Debug, Clone)]

hsmc-macros/tests/ui/t9_8_unknown_target.stderr

@@ -1,68 +1,13 @@
-warning: unexpected `cfg` condition value: `tokio`
-  --> tests/ui/t9_8_unknown_target.rs:8:1
-   |
- 8 | / statechart! {
- 9 | | M {
-10 | |     context: Ctx;
-11 | |     events: Ev;
-...  |
-17 | | }
-   | |_^
-   |
-   = note: no expected values for `feature`
-   = note: using a cfg inside a macro will use the cfgs from the destination crate and not the ones from the defining crate
-   = help: try referring to `statechart` crate for guidance on how handle this unexpected cfg
-   = help: the macro `statechart` may come from an old version of the `hsmc_macros` crate, try updating your dependency with `cargo update -p hsmc_macros`
-   = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg/cargo-specifics.html> for more information about checking conditional configuration
-   = note: `#[warn(unexpected_cfgs)]` on by default
-   = note: this warning originates in the macro `statechart` (in Nightly builds, run with -Z macro-backtrace for more info)
-
-warning: unexpected `cfg` condition value: `embassy`
-  --> tests/ui/t9_8_unknown_target.rs:8:1
-   |
- 8 | / statechart! {
- 9 | | M {
-10 | |     context: Ctx;
-11 | |     events: Ev;
-...  |
-17 | | }
-   | |_^
-   |
-   = note: no expected values for `feature`
-   = note: using a cfg inside a macro will use the cfgs from the destination crate and not the ones from the defining crate
-   = help: try referring to `statechart` crate for guidance on how handle this unexpected cfg
-   = help: the macro `statechart` may come from an old version of the `hsmc_macros` crate, try updating your dependency with `cargo update -p hsmc_macros`
-   = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg/cargo-specifics.html> for more information about checking conditional configuration
-   = note: this warning originates in the macro `statechart` (in Nightly builds, run with -Z macro-backtrace for more info)
-
-warning: unexpected `cfg` condition value: `journal`
-  --> tests/ui/t9_8_unknown_target.rs:8:1
-   |
- 8 | / statechart! {
- 9 | | M {
-10 | |     context: Ctx;
-11 | |     events: Ev;
-...  |
-17 | | }
-   | |_^
-   |
-   = note: no expected values for `feature`
-   = note: using a cfg inside a macro will use the cfgs from the destination crate and not the ones from the defining crate
-   = help: try referring to `statechart` crate for guidance on how handle this unexpected cfg
-   = help: the macro `statechart` may come from an old version of the `hsmc_macros` crate, try updating your dependency with `cargo update -p hsmc_macros`
-   = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg/cargo-specifics.html> for more information about checking conditional configuration
-   = note: this warning originates in the macro `statechart` (in Nightly builds, run with -Z macro-backtrace for more info)
-
 error[E0046]: not all trait items implemented, missing: `NonExistent`
-  --> tests/ui/t9_8_unknown_target.rs:19:1
+  --> tests/ui/t9_8_unknown_target.rs:21:1
    |
- 8 | / statechart! {
- 9 | | M {
-10 | |     context: Ctx;
-11 | |     events: Ev;
+10 | / statechart! {
+11 | | M {
+12 | |     context: Ctx;
+13 | |     events: Ev;
 ...  |
-17 | | }
+19 | | }
    | |_- `NonExistent` from trait
-18 |
-19 |   impl MActions for MActionContext<'_> {}
+20 |
+21 |   impl MActions for MActionContext<'_> {}
    |   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ missing `NonExistent` in implementation