`[Optional]` attribute for nullable-as-optional opt-in

[danfma]

Current behavior: nullable C# types (`string?`, `int?`, `T?` on a reference type) lower to TypeScript as `T | null` with a default of `null`. This is deliberate — see the `null vs undefined` memo — because APIs crossing the TS boundary benefit from the distinction between "this field is explicitly absent" (null) and "this field was not provided" (undefined).

However, some APIs (React props, JSON bodies that omit absent fields, Hono request schemas) want `undefined` semantics — the `param?: T` form where the key simply isn't set. Today there's no way to opt into that shape.

Goal

Introduce a `[Optional]` attribute that opts a parameter or property into the `param?: T` / `prop?: T` form instead of the default `param: T | null = null`.

Scope

• `[Optional]` attribute in `Metano.Annotations`
• Attribute usable on:
  • Method / constructor parameters (flips signature to `name?: T`)
  • Record / class properties (flips field to `name?: T`, no default)
  • Interface properties (`name?: T`)
• `TypeTransformer` / `InterfaceTransformer` / `RecordClassTransformer` read the attribute and emit the optional form
• Interaction with nullable types:
  • `string?` + `[Optional]` → `param?: string | null`
  • `string` + `[Optional]` → `param?: string`
• Tests covering every combination in `tests/Metano.Tests/NullableTranspileTests.cs` (or a new file)
• Documentation in `docs/attributes.md`

Default still wins

This is opt-in. The default stays `T | null = null` for safety at the API boundary.

Part of #14.

[danfma]

Spec mapping: Enhancement to FR-009 (nullability mapping). Proposes a new attribute [Optional] for opt-in param?: T instead of the default T | null = null.