chore: update alien-signals runtime integration

[aose-yuu]

Purpose

Update the alien-signals runtime integration for the 3.2 propagation contract.

Summary

• Updates alien-signals to ^3.2.1.
• Passes inner-write context to propagate() during signal writes from reactive effect execution.
• Keeps Sigrea-owned Link / ReactiveNode structural types local to avoid relying on d.ts-only upstream exports.
• Adds regression coverage for inner writes through computed chains and subscriber disposal during computed updates.

Verification

• pnpm -s cicheck