fix: make default filter read-only

[alyldas]

Summary:

• Made the shared default filter reject mutation methods while keeping check, censor, and analyze available.
• Kept factory-created and dictionary-created filters mutable for isolated runtime dictionaries.
• Updated API tests and README guidance for default-vs-factory usage.

Tests:

• npm test -- --run tests/api.spec.ts tests/curated-regression.spec.ts
• npm run check (using a temporary npm cache outside the user home)

Do not merge until maintainer approval.

Closes #115

[chatgpt-codex-connector]

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