Feat/ternary syntax

[jwulf]

This PR adds support for ternary syntax using the nullish coalescing operator ??.

New syntax: ?? null-coalescing operator

The ?? operator returns its left operand when that value is present, and
falls back to the right operand otherwise. It's intended for capability
effects and requires, typically to apply optional update parameters:

system Catalog {
  entity Product {
    owns: [
      title:       string,
      price_cents: uint,
    ]
  }

  capability update_product(
    product:     Product,
    title:       optional<string>,
    price_cents: optional<uint>
  ) {
    requires: [caller.role == "admin"]
    effects: [
      product.title       = title ?? product.title,
      product.price_cents = price_cents ?? product.price_cents
    ]
    sync: transactional
  }
}

title ?? product.title keeps the existing title when no new title is
supplied. It type-checks as the unwrapped (non-optional) type — e.g.
optional<string> ?? string yields string — and lowers to the JavaScript
null-coalescing operator in generated code:

const __effect_0 = await query(
  `UPDATE products SET title = $1, updated_at = NOW() WHERE id = $2 RETURNING *`,
  [(title ?? product?.title), req.params.id]
);

Scope / limitations

?? is not allowed in entity or top-level constraints, which compile to
SQL (use COALESCE there instead). Using it in a constraint is a compile-time
error:

entity Product {
  owns: [price_cents: optional<uint>]
  constraints: [(price_cents ?? 0) > 0]   // ❌ T013: '??' not allowed in constraints
}