fix(ts-profile): preserve fixed CodeableConcept codings

[sussdorff]

Summary

• generated TypeScript profile apply() methods now use a fixed-value merge helper instead of replacing fixed fields with Object.assign
• fixed CodeableConcept codings preserve compatible caller-provided secondary codings while adding missing fixed codings
• placeholder-only ValueSet expansions such as a sole UNK code no longer generate enum validation constraints
• the R4 bodyweight example now asserts factory equivalence with an apply() input that does not include extra preserved text

Validation

• bun test test/api/write-generator/typescript.test.ts
• bun run typecheck
• bun run lint (passes with existing warnings in profile-validation.ts and utils.ts)
• bunx biome check touched files
• bun examples/typescript-r4/generate.ts
• bun examples/typescript-us-core/generate.ts
• make test-typescript-r4-example
• bun run build
• attempted bun test; Bun 1.2.18 segfaulted after ~165s in the multi-package test run, with no assertion failure before the runtime crash

Context

This fixes the downstream FPDECoveragePrivatProfile/VWD failure mode generically in codegen rather than patching generated output.

[ryukzak]

Looks good, thank you!

Will be in a release in a few days.