fix: prevent vanilla share collisions across keys

[wobsoriano]

Summary

• fix share() to track a per-key serialized snapshot so unrelated store key updates do not rebroadcast on other shared keys
• keep existing $patch update path and avoid adding new runtime dependencies
• add a regression test that verifies sharing key a does not trigger a broadcast for shared key b

Verification

• vp test run src/__tests__/vanilla.test.ts
• vp lint src/vanilla.ts src/__tests__/vanilla.test.ts

Fixes #51