Implement RFC 0031: External type definitions (J2)

[fabriceHategekimana]

Tracking issue for rfcs/0031-external-type-definitions.md (accepted in #31).

Implementation checklist, copied from the RFC's own tail section — check items there as they land:

• typr-def.toml manifest parsing + format_version gate
• since/until added to FunctionMeta (stdlib_meta.rs) and to the manifest
• external .ty loading merged into standard_library.rs's context build, keyed by resolved typr.lock entries
• trust threshold + degrade-to-UnknownFunction at merge time
• typr types add|update|list|vendor, typr.lock read/write, digest verification, ~/.cache/typr/types/<pkg>/<digest>/
• cases/: missing definition, tier below trust, version below since, version above until, unreachable repository, digest mismatch — each asserting no hard error, only degradation
• [capabilities] gate enforced at fetch time (reject undeclared R; confirm-or---allow-r for declared)
• syntaxe.md — no lexeme changes expected, but confirm before merge
• Documentation PR on we-data-ch/typr.github.io (a How-to page for consuming an external definition; a Reference page for the manifest and #! keys), landing in the same release
• Implemented in: filled in in the RFC header

Corresponds to milestone J2 in typR/registry.md §13.

Co-Authored-By: Claude Sonnet 5 noreply@anthropic.com
Claude-Session: https://claude.ai/code/session_01ETPqhgWqDMt7UaUQXdxJmN