[TypeGen]: support external Zod schema overrides

[RejectKid]

Summary

Adds a Zod-specific property override for C# types whose runtime validator is supplied by a frontend package or shared schema module.

[TsType] remains responsible for static TypeScript typing. The new [ZodSchema] attribute supplies the runtime Zod expression and optional named import:

[TsType("Point", ImportFrom = "geojson")]
[ZodSchema("GeoJSONPointSchema", ImportFrom = "zod-geojson")]
public Point? DeliveryAreaPoint { get; init; }

The same configuration is available fluently for referenced models:

b.ForType<OrderDto>()
    .Property(x => x.DeliveryAreaPoint)
    .ZodSchema("GeoJSONPointSchema", "zod-geojson");

Generated output:

import { GeoJSONPointSchema } from 'zod-geojson';

export const OrderDtoSchema = z.object({
    deliveryAreaPoint: GeoJSONPointSchema.nullish(),
});

Behavior

• Replaces the inferred property validator, avoiding the current z.unknown() fallback.
• Supports [ZodSchema("expression", ImportFrom = "module")] and both fluent overloads.
• Supports inline expressions without imports, such as z.string().startsWith('ord_').
• Groups and deduplicates external named imports in file-per-class and single-file layouts.
• Preserves nullable, optional, read-only, and PatchField<T> modifiers after the override.
• Treats the explicit schema as authoritative instead of appending inferred format or validation constraints.
• Removes overridden C# property types from internal schema dependency, cycle, and topological-order analysis.
• Works with z.compile(...), inferred types, generated guards, and TanStack payload validation.

The documentation also explains that z.toZod<T>() requires exact type equality. A schema merely assignable to a [TsType] type may not satisfy conformance. For zod-geojson, its own GeoJSONPoint and GeoJSONFeature inferred aliases support exact conformance.

Closes #20.

Affected package(s)

• ZibStack.NET.TypeGen
• ZibStack.NET.TypeGen.Abstractions
• TypeGen sample and documentation

Type of change

• Bug fix
• New feature
• Generator / generated-code behavior change
• Analyzer / diagnostic change
• Docs only
• Build / tooling

Examples and documentation

• The sample demonstrates attribute and fluent overrides using zod-geojson.
• The package README includes installation and client syntax.
• The Zod emitter documentation covers generated imports, inline expressions, modifier behavior, inference, and conformance.
• Type-mapping and configurator documentation distinguish static [TsType] overrides from runtime [ZodSchema] validation.

How was this verified?

• dotnet test ... --filter "FullyQualifiedName!~CompilationTests" — 262 passed.
• Zod compilation tests — 7 passed, including real zod@4.6.1, zod-geojson@1.7.1, @types/geojson, and tsc compilation.
• TypeScript compilation tests — 2 passed.
• Focused emitter, parser, and configurator tests — 63 passed.
• TypeGen sample rebuilt successfully and generated the expected GeoJSONPointSchema / GeoJSONFeatureSchema imports and usages.
• dotnet build ZibStack.NET.slnx --no-restore — succeeded with 0 errors and no new warnings.

The Astro documentation build was attempted, but the existing local docs/node_modules contains files that Windows reports as access denied. No documentation-source error was reported before that environment failure.

Checklist

• Branched from master, focused on a single change
• Tests added or updated (generator behavior asserted where applicable)
• Docs updated under docs/src/content/docs/ and the package README
• Build is clean — no new warnings
• Commit subjects use the [Package]: … prefix