fix: avoid compile-time deps from schema function options (#38)

[bamorim]

Problem

many_to_many(:reviews, Review, join_through: Book) inside typed_schema created a compile-time dependency on Book, while the identical schema with plain Ecto.Schema does not (#38, reproduced in axelson/typed_ecto_schema_repro). This causes recompilation cascades in user projects.

Root cause

The syntax sugar for schema functions injected the full options list, unescaped, into the generated TypeBuilder.add_field/5 call:

unquote(TypeBuilder).add_field(__MODULE__, ..., unquote(Macro.escape(type)), unquote(opts))

The type was escaped (PR #18), but opts was not — so every option value was evaluated in the module body at compile time, and a module alias in an option like join_through: resolved into a compile-time dependency. (Ecto itself avoids this by expanding such aliases under a fake function env.)

Fix

Forward only the options add_field/5 actually reads — :null, :enforce, :default, :values, :__typed_ecto_type__, and the belongs_to-specific :define_field/:foreign_key/:type — mirroring how the polymorphic embeds support already handles this. Notes:

• The forwarded values keep their original AST (not escaped), so values: @attr still resolves to the real list in the module body — the additional_types feature and enum type inference keep working.
• The AST-variable opts path (issue 0.4.2 breaks usage in defmacro #52) does the same Keyword.take at runtime.
• The full original options still go to the re-emitted Ecto call untouched (minus the enhanced keys, as before).
• The custom-Ecto.Type case from the issue (field(:country, Country)) is untouched: that compile-time dependency is inherent to Ecto (it exists with plain Ecto.Schema too).

Regression test (#26)

Adds test/typed_ecto_schema/compile_deps_test.exs: it compiles a schema from a string with a compiler tracer attached and records every module referenced at compile time (env.function == nil, which is what makes mix xref draw a compile edge). It asserts the join_through: module is never referenced at compile time, while a custom Ecto.Type used with field is — proving both that the tracer setup detects deps and that the Ecto-inherent case stays untouched. The test fails on master and passes with this fix.

Verification with mix xref

Throwaway project using this branch as a path dep, with two schemas — TypedUser (typed_schema) and PlainUser (plain Ecto.Schema), both declaring many_to_many(:reviews, XrefRepro.Review, join_through: XrefRepro.Book) in separate files.

Before (master):

$ mix xref graph --label compile
lib/xref_repro/typed_user.ex
└── lib/xref_repro/book.ex (compile)

After (this branch):

$ mix xref graph --label compile
(no output — no compile edges, matching plain Ecto)

mix test (66 passed), mix credo --strict, and mix dialyzer all pass.

Closes #38
Closes #26

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