Design: dist/tool/extension layout, manifests (JSONC), resolution (global vs local)

**Summary**
Define structure, manifest schema (JSONC with `.json` extension), RID naming, precedence rules, config keys, and fallback policy for dist/tool/extension management. Align with layered config (ref: #294). Produce a short design doc/PRD update.

**Scope**
- Folder layout: global vs local roots; RID naming (e.g., `linux-x64`, `osx-arm64`, `win-x64`).
- Manifest schema (JSONC): fields for name, version, platform, source URL, optional hash/metadata.
- Resolution precedence: project-local (if opted) → user/global default → error; per category.
- Config keys for active selections and download locations.
- Fallback policy: behavior when local selection missing—error vs optional fallback to global (decide and document).
- Output: design doc + PRD/notes update.

**Acceptance**
- Approved design covering layout, manifests (JSONC), precedence, config keys, fallback policy.
- Decision on RID naming convention and minimal integrity check requirements.
- Notes added to PRD/implementation notes.