feat: Support grounding without parent_ids

[nicoloesch]

Summary

Grounding currently requires one or more explicit parent_ids to anchor candidates within a known hierarchy. That precision has a cost: it makes grounding unusable in contexts where no such anchor exists yet

• annotating free text with no known ontological context
• benchmark cases sourced from external tools that don't carry a parent, or
• open-ended graph exploration (MCP/ACP) where a caller is searching the concept space itself rather than validating a hit against an already-known branch.

This PR adds a parent-less mode: candidates still resolve and standardize to their canonical OMOP form via lexical/semantic search and identity hops, just without ancestor verification. This trades disambiguation power for coverage where no anchor is available.

• Fixes Grounding without Parent ID #22

Implementation

• find_standard_paths now accepts targets: Optional[Tuple[int, ...]]
  • With targets: behavior is unchanged (batched ancestor check, separation = ancestor-hierarchy distance to the required parent).
  • Without targets: the first standard concept reached via an edge (IDENTITY in grounding) is accepted directly
    • no ancestor query at all
    • separation repurposed as the identity-hop distance from the original candidate
    • expansion capped at max_depth hops to bound the walk
• Added StandardConcept.identity_hops: the raw BFS hop count, captured unconditionally in both modes from item.iterations (previously computed and discarded). Currently unused but allows disambiguation in the constrained case
• Scoring itself (scoring.py) needed no changes: parsimony_penalty = alpha * separation already generalisation correctly since separation carries the right distance for either mode.
• Deliberately did not add a depth/specificity-to-root signal.
  • The concept graph has no single root. Tt's a DAG with many disconnected per-domain/per-vocabulary top nodes
  • depth-to-root isn't comparable across candidates (a Condition concept vs. a Drug concept, or even two branches of the same domain) and would read as a principled signal while actually being noise.