hygm: ship the generic default model learned ontologies start from

[antejavor]

First build step of map #431, "An ontology that learns from the user's sessions". The content was decided in #436.

What

• hygm.default_model() loads a packaged default_ontology.yaml:
  • fixed core: User, Person, and the value types Duration, Quantity, Money, Date, TimeWindow;
  • generic layer: Organization, Location, Event;
  • catch-alls: Topic, Artifact;
  • relations: 10 generic user-centred ones, plus 7 value relations, so every value type has at least one relation into it (the gap Run the revised derivation contract: batched propose and consolidate #372 hit).
• CORE_LABELS, VALUE_LABELS, CATCH_ALL_LABELS: the label sets that derivation (never merges or retires the core) and the adoption gate (measures catch-all share, Grill: the automatic gate that adopts a new model version #435) will read.
• with_core(model): adds any core type missing from a user-supplied schema, for derive = "extend".

Choices worth a look

• Artifact is global: a tool or library named in many sessions is one node. The backend's existing guard keeps all-lowercase mentions of a global type per chunk, so generic nouns ("the file") don't become hubs.
• Topic stays per chunk: as the high-volume catch-all, capitalized subjects would turn into hubs.
• Unchanged: unstructured2graph's own DEFAULT_ONTOLOGY, as decided in Grill: the default ontology and a user-supplied one #436.
• Version: hygm stays at 0.1.0; the bump goes with the release, once sessions-graph uses the default.

Tests

hygm/tests/test_default.py checks that:

• the default passes validate_model();
• it holds the core and the catch-alls;
• every value type is per-mention (span) and has a relation into it;
• every relation constrains both of its endpoints;
• Artifact is global and Topic per chunk;
• with_core adds only the missing core types.

All 41 hygm tests pass, and ruff and ty are clean. The YAML is confirmed to ship in the built wheel.