refactor: add template-backed simulation codegen

[jayscambler]

Summary

• move simulation-family code generation onto a template-backed rendering path
• add a shared placeholder renderer with unresolved-token protection
• keep generated simulation behavior stable while reducing giant inline string construction in the generator

TDD / DDD / DRY framing

This is the next stacked architectural PR after #653.

Bounded extraction:

• Simulation Codegen Rendering — isolate template rendering from simulation spec normalization/mapping

Test-first work:

• added a template renderer test first
• added a simulation codegen test asserting all placeholders resolve and the output remains executable
• implemented the renderer and moved simulation code shape into a dedicated template module

Changes

New modules

• ts/src/scenarios/codegen/template-renderer.ts
• ts/src/scenarios/codegen/templates/simulation-template.ts

Updated module

• ts/src/scenarios/codegen/simulation-codegen.ts
  • now builds replacements and renders the dedicated template instead of assembling one giant inline string directly

New tests

• ts/tests/simulation-codegen-template.test.ts
  • deterministic placeholder rendering
  • simulation codegen emits fully rendered executable source

Validation

• cd ts && npx vitest run tests/simulation-codegen-template.test.ts tests/codegen.test.ts tests/codegen-runtime.test.ts
• cd ts && npm run lint

Why this matters

This addresses the remaining code-review recommendation to move away from large monolithic codegen strings. It also establishes a reusable rendering seam for future family-by-family migration to template-backed generation.