TypeSchema: better error + skip R5 types in R4-target extensions

[ryukzak]

Context

Generating against kbv.ita.for@1.3.1 together with de.basisprofil.r4@1.6.0-ballot2 pulls in the transitive dep hl7.fhir.uv.extensions.r4@5.2.0. That package ships R4-targeted extensions whose value[x] choices are typed as R5-only datatypes (CodeableReference, Availability). On R4 generation those types do not resolve — codegen aborts with an unhelpful single-line error.

Changes

TypeSchema: improve field type resolution error

Replace the single-line throw in field-builder.ts with a structured multi-line message and an R4-context-gated hint.

Before

Could not resolve field type: <http://hl7.org/fhir/StructureDefinition/biologicallyderivedproduct-manipulation>.valueCodeableReference: <CodeableReference> (pkg: 'hl7.fhir.uv.extensions.r4#5.2.0'))

After

Could not resolve field type:
  package: hl7.fhir.uv.extensions.r4#5.2.0
  schema:  http://hl7.org/fhir/StructureDefinition/biologicallyderivedproduct-manipulation
  field:   valueCodeableReference
  type:    CodeableReference
  hint:    'CodeableReference' is an R5+ type and is not available when generating against R4.
           Either skip this canonical via skip-hack.ts, or upgrade the target to R5.

The hint only fires when:

• element.type is in R5_ONLY_TYPES (Availability, CodeableReference, ExtendedContactDetail, MonetaryComponent, RatioRange, VirtualServiceDetail), AND
• the failing schema's package transitively depends on hl7.fhir.r4.core (otherwise the hint would mislead R5 generation).

TypeSchema: skip biologicallyderivedproduct R5-typed extensions

Two more entries in src/typeschema/skip-hack.ts for hl7.fhir.uv.extensions.r4:

• biologicallyderivedproduct-manipulation
• biologicallyderivedproduct-processing

Both reference CodeableReference. Joins the existing six entries (extended-contact-availability, immunization-procedure, specimen-additive, workflow-barrier, workflow-protectiveFactor, workflow-reason).

TypeSchema: extend kbv-r4 example with de.basisprofil + kbv.basis

Update examples/on-the-fly/kbv-r4/generate.ts to pin the cross-package version graph that surfaced the issue:

-    .fromPackage("hl7.fhir.r4.core", "4.0.1")
     .fromPackage("kbv.ita.for", "1.3.1")
+    .fromPackage("de.basisprofil.r4", "1.6.0-ballot2")
+    .fromPackage("kbv.basis", "1.8.0")

hl7.fhir.r4.core is now pulled transitively. The example now exercises the R4/R5-extensions interaction so future regressions are caught.

Notes

• The skip list remains a hardcoded URL allowlist — long-term we want to detect R5-only types automatically. This PR keeps the existing mechanism and only adds the missing entries needed to unblock current users.
• Related discussion: TypeSchema: fix versioned canonical resolution for cross-package base types #150, fix(register): resolve cross-package + transitive-version + cache-mismatch canonical lookups #151.