fix: include serialization error details

[KirtiRamchandani]

Problem

When Comlink catches an error while serializing a return value, the caller only receives Unserializable return value. That hides the underlying serialization error message that usually explains what could not be cloned or serialized.

Root cause

The serialization fallback catch block discarded the caught error and always created the same generic TypeError.

Solution

Build the fallback TypeError message from the caught serialization error when it has details, while keeping the existing generic prefix. Added a same-window regression test with a deterministic transfer handler that throws during serialization.

Tests

• npm run build
• npm run test:types
• CHROME_ONLY=1 npm run test:unit
• npm run test:node
• npm exec --yes prettier@2.8.3 -- --check src/comlink.ts tests/same_window.comlink.test.js
• git diff --check

Fixes #679.