Detects SCXML features used in documents so the test harness can fail precisely on unsupported features.

Test-side surface for chart authors, versioned with the engine: no module in lib/ outside Statifier.Testing.* may reference anything inside it, so the engine never consults feature detection to decide behavior (ADR-0053, amending ADR-0006). A test whose document uses an unsupported feature flunks with that feature named, so it can never masquerade as passing (see docs/testing.md, ADR-0006).

Detection is regex-based over raw XML. v2 has no parsed document struct yet; when one lands, a detect_features/1 clause for it is added alongside the string clause.

The registry reflects what v2 actually supports today: basic/compound/parallel/final/history states, the initial attribute and <initial> element, event/eventless/targetless/internal/wildcard transitions, <onentry>/<onexit>, <raise>, <log>, static <donedata>, <datamodel>/<data>, <assign>, cond-guarded transitions (including <if>/<elseif>/<else>), <foreach>, and <script> (:partial - see the registry entry), <send>/<invoke>/ <cancel>/<finalize> and their attributes. script_elements is the only entry that is not :supported; see feature_registry/0 for the authoritative, up-to-date list.