Support subsetOf() and supersetOf() FHIRPath functions

Spec link: https://hl7.org/fhirpath/#subsetofother--collection--boolean and https://hl7.org/fhirpath/#supersetofother--collection--boolean

Split out of #2385, which implemented the other 7 FHIRPath existence functions (all, allTrue, allFalse, anyTrue, anyFalse, isDistinct, distinct).

Why these two are separate

subsetOf(other : collection) and supersetOf(other : collection) each take a plain collection-typed argument that must be evaluated against the surrounding iteration focus ($this), not against %context (the root resource) — per the FHIRPath spec's general argument-evaluation rule (only expression-typed parameters like where/select/all's criteria get their own $this scope; everything else, including collection-typed arguments and operator operands, inherits whatever $this already is). This is confirmed empirically by the fhirpath.js reference implementation (src/fhirpath.js, makeParam, the "AnyAtRoot"/"Any" branch: const $this = ctx.$this || ctx.dataRoot;) — the same code path used by subsetOf, supersetOf, union, combine, and every infix operator.

Pathling's FunctionParameterResolver.resolveArgument (fhirpath/src/main/java/au/csiro/pathling/fhirpath/function/resolver/FunctionParameterResolver.java) instead evaluates plain Collection-typed arguments against evaluationContext.getInputContext(), which is fixed at construction to the root resource for the whole query — not the current iteration item. Concretely:

Patient.name.select(('James').subsetOf(given))

resolves given against the root Patient, not the current name item, because given doesn't exist on Patient this returns the wrong result rather than the per-name given array.

This is not a new problem: it's the exact same gap that union()/combine() (#2384, PR #2587) hit and deliberately worked around via parser-level desugaring into EvalOperator ASTs rather than fixing the resolver. See the archived design doc at openspec/changes/archive/2026-04-11-add-combining-functions/design.md for the full write-up, including the explicit Non-Goal: "Refactoring Composite/EvalFunction/FunctionParameterResolver to propagate an 'outer focus' through chained expressions. That is a broader architectural change that would benefit future Collection-argument functions but is out of scope for this issue."

subsetOf/supersetOf are the first functions since union/combine to need this — unlike union, there's no existing operator to desugar into, so this would need a new operator (or pair of operators, since subsetOf/supersetOf aren't the same relation reversed at the AST level the way combine mirrors |) plus a parser desugaring rule, following the same pattern CombineOperator established.

Scope

• Design a fix (or accept the same operator-desugaring pattern) for subsetOf(other) and supersetOf(other), ensuring x.subsetOf(y) and x.supersetOf(y) correctly resolve y against the enclosing iteration focus, matching the spec's worked examples and the fhirpath-js reference corpus (fhirpath/src/test/resources/fhirpath-js/cases/5.1_existence.yaml, sections 5.1.9/5.1.10).
• This is a FHIRPath-framework-level change (touches parser/ and/or operator/), so per this project's implement-pathling skill it should go through an OpenSpec change before code exists, the same way #2587 did.

Implementation reference

A draft, spec-correct ExistenceLogic.subsetOf(Collection, Collection) helper (forall/exists over the comparator-aware equality machinery, with the empty-collection short-circuit rules from the spec) was written and verified correct in isolation while implementing #2385, then reverted once the argument-scoping bug was found — see that PR's history for the reasoning and a working reference implementation of the membership-testing logic itself (the part that isn't affected by this gap).

Start with FunctionParameterResolver.java, the parser/ and operator/ implementations, and the archived combine design at openspec/changes/archive/2026-04-11-add-combining-functions/design.md. Compare the fhirpath-js cases in fhirpath/src/test/resources/fhirpath-js/cases/5.1_existence.yaml with the existing CombineOperator pattern and the reverted #2385 implementation. Done means subsetOf() and supersetOf() resolve collection arguments against the enclosing iteration focus and pass the relevant spec examples.