Make $$schema stable across module reloads

[agcty]

Fixes #100.

Summary

• make $$schema accept schemas created before a module reload
• add a regression test that imports schema.js twice via distinct module URLs
• keep the fix local to schema recognition without changing the global schema cache

Why this approach

schemaStore is global, so $type("...") can return a schema instance created by an earlier evaluation of schema.js.
After a module reload, that cached schema no longer passes instanceof Schema against the new constructor, which causes $() / $union(...) to throw Unexpected case.

This patch makes schema recognition reload-stable by adding a global symbol marker to Schema.prototype and accepting that marker in $$schema in addition to instanceof Schema.

A broader redesign around branding all schema objects more explicitly may also make sense, but that would be a larger architectural change. This PR keeps the fix minimal and targeted to the actual reload mismatch.

Verification

• node --unhandled-rejections=strict ./src/test.js --filter 'schema: schema reload stability'
• npx standard src/schema.js src/schema.test.js