fix: add nullable support and replace silent failures with explicit errors

[ssilvius]

Summary

• Nullable/nullish support: z.nullable(), z.nullish(), and z.string().nullable().optional() now correctly unwrap and set isNullable on field descriptors. Previously these crashed or produced wrong output.
• Fail-hard philosophy: Replaced 7 silent failure patterns with explicit throws. The code generator now fails loudly instead of silently producing partial/incorrect forms.
• Unknown check warnings: Zod checks not recognized by the constraint extractor are surfaced as warnings instead of being silently ignored.

What changed

File	Change
unwrap.ts	Handle nullable wrappers, return isNullable flag
introspect.ts	Exhaustive switch in buildMetadata, empty enum guard, unknown checks, description from outer wrapper
checks.ts	Collect unknown Zod checks into warnings array
generator.ts	Remove try-catch that silently dropped fields
form-wrapper.ts	Throw on missing config instead of silent skip
field-components.ts	Validate non-empty options for Select/RadioGroup
cli.ts	Proper type narrowing, no silent default export fallback
types.ts	Add isNullable to FieldDescriptor, warnings to FormDescriptor

Test plan

• 242/242 tests passing (was 238 before nullable tests added)
• 4 new nullable test cases in introspect.test.ts
• Updated existing tests to match fail-hard behavior (generator, form-wrapper)
• Biome lint clean
• TypeScript typecheck passes

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