docs(charter): admit one opt-in C++ engine behind the Three.js API

[jonit-dev]

TL;DR: the charter now admits one owned C++ engine behind the Three.js API, opt-in per game, with the legacy host as the default until it proves itself. Owner decision 2026-10-12. Docs only. This is slice 0 of the gradual native-engine merge.

Why

The native engine (feat/native-engine, PR #438) is merged into develop in small slices. Each slice must pass develop's gates on its own. Without this amendment, every engine slice breaks root rule 5 ("an IR") and the charter row "a second renderer".

What changed

• docs/architecture/CHARTER.md
  • §2: the ban is now on an authored IR. The shader IR is internal.
  • §2: "A second renderer" names the one exception, the opt-in C++ engine.
  • §7: "The default runtime is a host, not a renderer". The opt-in engine is the exception.
  • §11.1: same wording as §2.
  • §6b: the CSS line says "also an exception", because there are now two.
• docs/architecture/CHARTER-HISTORY.md: one amendment row.
• Root AGENTS.md rule 5 names the exception. CLAUDE.md is regenerated.
• docs/architecture/NATIVE-ENGINE-DECISION.md is new. It is the batch's decision record, with three changes:
  • the parked decisions on compiling game code (Perry, gate T) are removed;
  • links to PRDs that are not on develop yet are plain text;
  • legacy is the default until the owner promotes the engine (decision 10).

NATIVE-RUNTIME.md and packages/runtime-native/AGENTS.md are unchanged. They describe the shipped host, and primary docs name only shipped things.

Evidence

• pnpm check:docs: pass
• pnpm sync:agents --check: pass
• vitest on check-doc-links, evidence-budget, evidence-citations, sync-agent-docs, ci-structure, ci-needs and primary-docs specs: 7 files, 254 tests pass

Next slices

Slice 1 is a separate PR: core fixes that need no engine. The engine C++ slices come after this PR merges.

[jonit-dev]

Merged into #493: the epic now has one PRD and one PR. The charter commit is on #493 as fc261bb.