feat(health): initial stable release v0.9.0

HTTP health check handler with parallel goroutine-per-check execution, 5 s request-derived timeout, and two-level criticality (LevelCritical → 503, LevelDegraded → 200).

What's included:
- `Checkable` interface (HealthCheck / Name / Priority) and `Level` type with LevelCritical and LevelDegraded constants
- `NewHandler(logger, checks...)` returning http.Handler; runs all checks concurrently via buffered channel, returns JSON with per-component status and latency
- `ComponentStatus` and `Response` types for the JSON response body

Tested-via: todo-api POC integration
Reviewed-against: docs/adr/

compliance_test.go

@@ -0,0 +1,27 @@
+package health_test
+
+import (
+	"context"
+
+	"code.nochebuena.dev/go/health"
+)
+
+type testLogger struct{}
+
+func (t *testLogger) Debug(msg string, args ...any)                   {}
+func (t *testLogger) Info(msg string, args ...any)                    {}
+func (t *testLogger) Warn(msg string, args ...any)                    {}
+func (t *testLogger) Error(msg string, err error, args ...any)        {}
+func (t *testLogger) WithContext(ctx context.Context) health.Logger   { return t }
+
+// Compile-time check: testLogger satisfies health.Logger.
+var _ health.Logger = (*testLogger)(nil)
+
+type testCheck struct{}
+
+func (t *testCheck) HealthCheck(ctx context.Context) error { return nil }
+func (t *testCheck) Name() string                          { return "test" }
+func (t *testCheck) Priority() health.Level                { return health.LevelCritical }
+
+// Compile-time check: testCheck satisfies health.Checkable.
+var _ health.Checkable = (*testCheck)(nil)