fix(errors): replace chained cast on FACTORY_SYMBOL with typed property

[martyy-code]

Closes #71.

What

error.ts was assigning the factory marker via a chained type assertion:

(instance as unknown as Record<typeof FACTORY_SYMBOL, () => unknown>)[FACTORY_SYMBOL] = ErrorFactoryInstance;

This is the canonical example of the pattern rule 0008 forbids. The librarys own source was a counter-example to its own doctrine.

How

• Move FACTORY_SYMBOL from error.ts (implementation) to types.ts (contract). The marker is part of the type — it belongs with the type.
• Declare [FACTORY_SYMBOL]: ErrorFactory<TFields> on ErrorInstance<T>. The marker is now a typed property.
• The assignment becomes a single property write on the declared type. The as unknown cast is gone.
• is/index.ts updates its import to point at types.ts (where the symbol now lives).
• Add a regression test confirming the marker is attached to a fresh instance and points back to the producing factory.

Verification

• 83/83 tests pass (82 → 83, new regression).
• pnpm type-check clean.
• pnpm lint clean (run from the package dir).
• Public API unchanged; the symbol is internal.

Note

The pre-commit eslint --fix hook is broken: it runs from the monorepo root and cannot find eslint.config.js (which only lives in packages/errors/). I committed with --no-verify after confirming the code passes prettier + eslint when run from the package dir. Worth a separate PR to fix the hook.

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