feat(dang): add reflection-driven Go FFI

[vito]

What

Adds GoFFI, a reflection-driven bridge that exposes arbitrary Go types
and values to Dang code: it derives a Dang Module from a
reflect.Type, wraps a live Go value as a Dang Value, and invokes Go
methods from Dang. This implements phases 1–2 of go-ffi.md (the
Dang-side MVP plus interface bridging).

It's a sibling to the GraphQL/Dagger import path — same target (Dang's
Module/ModuleValue/Value model), different source — so embedders
can expose in-process Go types to scripting code without routing through
GraphQL.

Public API

• GoFFI (zero value usable) with a configurable NameMapper.
• Converter{DangType, ToDang, FromDang} + RegisterConverter, for
  bridging interfaces and host-specific types.
• RegisterType(env, reflect.Type) (*Module, error) — idempotent and
  recursion-safe (caches the *Module before walking fields).
• WrapGoValue(any) (Value, error).
• DefaultNameMapper (PascalCase → camelCase; URL→url,
  HTMLBody→htmlBody).

WrapGoValue produces a *ModuleValue with lazy field entries and
Callable methods, so field access, method binding, and type checking
come for free from the existing Select.Eval machinery.

Mapping rules

Primitives, pointers (nullable), slices, and structs (recursively
registered) map by default. Interfaces require a registered Converter.
Trailing error returns raise a BasicError. Void mutators and
multi-return methods are skipped so registering arbitrary types stays
robust.

Note

Go reflection cannot recover parameter names, so method arguments are
named positionally (arg1, arg2, …). No-arg methods — the common
Booklit case — are unaffected and remain auto-callable.

Tests

10 tests cover the name mapper, type-derivation schemes, field access
and auto-called/argument'd method calls through the full
parse→infer→eval pipeline, (T, error) raising and catching, recursive
types, the unregistered-interface error, an interface Converter, and
the wrap-before-register error.

Phases 3–4 (Booklit integration) are separate, consumer-side follow-ups.