Add slot() function and hide gating for derived slot bindings

[amc-corey-cox]

Summary

• Expressions can now call slot('name') to reference previously computed slot values, enabling multi-stage derivations where value_mappings handles declarative lookups and expr handles computation
• Slots with hide: true are computed (available to downstream slot() calls) but excluded from output
• Hidden slots skip target-schema post-processing (cardinality coercion, range mapping) since they don't exist in the target schema

Motivation

Without slot(), combining value_mappings with expr is impossible — users are forced to inline everything into case() expressions that are unreadable and structurally irreversible:

# Before: unreadable inline case()
visit_site:
  expr: "case({visit_code} == '1', 'SCREENING', ...) + ' at ' + case({site_code} == 'A', 'Boston', ...)"

# After: declarative lookups + expression composition
_visit_label:
  hide: true
  populated_from: visit_code
  value_mappings:
    "1": "SCREENING"
    "7": "BASELINE"
visit_site:
  expr: "slot('_visit_label') + ' at ' + slot('_site_label')"

Design decisions

• Insertion order: Slot derivations are evaluated in declaration order. slot() references must appear after the referenced slot. No topological sort — YAML authors naturally write top-to-bottom.
• Compute-then-filter: All slots (including hidden) are computed into tgt_attrs, then hidden slots are stripped before returning. This keeps slot() uniform regardless of whether the referenced slot is hidden.
• Injection via functions param: slot is a lambda closing over tgt_attrs, threaded through existing functions plumbing in eval_expr_with_mapping. No changes to Bindings.

Test plan

• slot() basic: reference a previously derived slot value
• Hidden slot suppression: hidden slots computed but excluded from output
• Hidden slot with value_mappings: declarative lookup → slot() reference pattern
• slot() for missing name returns None
• Integration test: all features compose correctly
• Full test suite: 592 passed, no regressions

Closes #164

[Copilot]

Pull request overview

Adds support for referencing previously-derived target slot values during object transformation by introducing a slot('name') expression function, and implements hide: true for slot derivations so intermediate computed slots can be used internally but excluded from output.

Changes:

• Extend expression evaluation to accept caller-injected functions (used to provide slot()).
• Evaluate slot derivations in declaration order and support hidden (non-output) derived slots.
• Add unit/integration tests covering slot() and hidden-slot output suppression.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/linkml_map/transformer/object_transformer.py	Injects slot() into expression evaluation, computes hidden slots for downstream use, and strips hidden slots from final output.
src/linkml_map/utils/eval_utils.py	Adds optional functions parameter to eval_expr_with_mapping to merge caller functions with built-ins.
tests/test_transformer/test_object_transformer_new.py	Adds test scaffolds and assertions for slot() behavior and hide: true output exclusion.