Preserve content references in shared snapshots

[caoergou]

Summary

• inline successfully read contentRef bodies into shared snapshots
• retain the originating content-pack ID and deterministic asset-manifest fingerprint for unread references
• reject imports atomically with structured, named errors when a pack, manifest, or asset is missing
• keep the public ContentResolver contract backward compatible through a read-aware extension
• document the portability and trust semantics

Why

A shared save previously retained lazy content references without enough information to reproduce what the sender had already read or diagnose an unavailable unread asset. This made saves dependent on ambient content-pack state.

Validation

• npm run typecheck
• focused ESLint and Prettier checks on all changed source/test files
• 37 focused snapshot and ContentRef tests
• 40 scenario, deep-link, persistence, and ContentRef regression tests
• npm run schema:check
• npm run knip
• npm run guard:nocheck
• npm run guard:purity
• npm run scenario:ci
• npm run build:lib
• npm run scenario:build
• npm run size:check

Closes #266