Dependency type split

• SpecializationTypeSchema.dependencies narrowed from TypeIdentifier[] to Identifier[] — nested identifiers are filtered out
• ProfileTypeSchema.dependencies stays TypeIdentifier[] — keeps nested identifiers
• Split extractDependencies into two functions:
  • extractDependencies — filters out nested, returns Identifier[] (for specializations)
  • extractProfileDependencies — keeps nested, returns TypeIdentifier[] (for profiles)
  • Shared collectRawDeps helper extracts the common logic
• Split transformFhirSchema into profile/specialization branches with correct dep extraction
• Split treeShakeTypeSchema dependency recalculation by schema kind

Specialization dependencies no longer include nested identifiers:

// Before
dependencies: [
    { kind: "resource", url: ".../CodeSystem" },
    { kind: "nested", url: ".../CodeSystem#concept" },
]

// After (specialization)
dependencies: [
    { kind: "resource", url: ".../CodeSystem" },
]

// After (profile) — nested kept
dependencies: [
    { kind: "resource", url: ".../CodeSystem" },
    { kind: "nested", url: ".../CodeSystem#concept" },
]

Type safety improvements

• Replace schema.identifier.kind checks with type predicates (isResourceTypeSchema, isSpecializationTypeSchema, isLogicalTypeSchema)
• Remove as unknown as SpecializationTypeSchema casts in writers — NestedTypeSchema is structurally assignable
• Remove as unknown as TypeSchema casts for nested schemas
• Replace as ProfileIdentifier cast with assert(isProfileIdentifier(...))
• Remove dead isNestedIdentifier/kind === "nested" branches in TS writer
• Make groupByPackages generic to preserve input types, removing as Record<...> casts
• Widen mutableSelectFields/mutableIgnoreFields to accept SpecializationTypeSchema | ProfileTypeSchema
• Make concatIdentifiers generic (<T extends TypeIdentifier = TypeIdentifier>)
• Narrow extractDependencies param to Identifier, extractProfileDependencies to ProfileIdentifier
• Narrow base to Identifier | undefined in transformFhirSchema with assert
• Use const arrow style for collectRawDeps, extractDependencies, extractProfileDependencies

Bug fix: constrained choice on nested types

• Add nestedIndex fallback in resolveByUrl so constrainedChoice resolves nested types
• Blood Pressure profile component slices now correctly detect value[x] → valueQuantity constraint:

// Before — all value[x] variants still exposed
export type Observation_bp_Component_SystolicBPSliceFlat = Omit<ObservationComponent, "code">;

// After — choice variants stripped, intersected with constrained type
export type Observation_bp_Component_SystolicBPSliceFlat =
    Omit<ObservationComponent, "code" | "value" | "valueQuantity" | ...> & Quantity;