TypeSchema: followReferences tree-shake option to generate referenced types

[ryukzak]

• Add followReferences?: boolean to TreeShakeRule (default: false): a root with the flag adds its reference targets — base resources (reference.resource) and target profiles (reference.profiles), including nested-type fields — to the kept set, with their regular dependencies pulled transitively.
• Following is non-transitive over references: pulled-in types keep their own reference targets as plain string literals, so one flag cannot drag in the whole package.
• Explicit tree-shake roots win over followed targets, so a followed reference cannot override another root's selectFields/ignoreFields rule.
• Add treeShakeDefaults: { followReferences } to IrConf to enable the option for every root at once; a rule's own value overrides the default.
• Tests: default drops targets, per-rule and default-based following, rule-over-default precedence, non-transitivity.
• Document both options in README (Tree Shaking section) and CLAUDE.md; clarify that reference targets are not dependencies by default.

Follow-up to #201: reference targets no longer need to be tree-shake roots for correct Reference<...> types, but users may still want the referenced types generated — this makes that a one-flag opt-in instead of enumerating every target:

// before: every referenced type enumerated by hand
.typeSchema({
    treeShake: {
        "kbv.basis": {
            "https://fhir.kbv.de/StructureDefinition/KBV_PR_Base_Condition_Diagnosis": {},
            "https://fhir.kbv.de/StructureDefinition/KBV_PR_Base_Patient": {},
        },
    },
})

// after
.typeSchema({
    treeShake: {
        "kbv.basis": {
            "https://fhir.kbv.de/StructureDefinition/KBV_PR_Base_Condition_Diagnosis": { followReferences: true },
        },
    },
})

// or for all roots at once
.typeSchema({
    treeShake: { ... },
    treeShakeDefaults: { followReferences: true },
})