Manifest-driven field selection for Lua-side queries.

Translates a fields tree supplied by a Lua script into the three things Ash needs:

• select — the list of attribute atoms to pass to Ash.Query.select/2
• load — the load statement to pass to Ash.Query.load/2 (or Ash.Changeset.load/2)
• template — an opaque structure the encoder walks to emit only the requested fields

The Lua side accepts a list of strings and one-key maps:

fields = {
  "id", "title",
  { author = { "id", "name" } },                          -- nested relationship
  { comments = { "id", "body" } },                        -- has_many
  { title_prefixed = { args = { prefix = "x: " } } },     -- calc with args
  { metadata = { "priority", "category" } },              -- typed map sub-selection
  { coordinates = { "latitude", "longitude" } },          -- typed tuple
  { content = { text = { "body" } } },                    -- union member sub-selection
}

Default (no fields provided) returns primary-key attributes only for resource records. For non-resource return types (typed map, tuple, primitive, etc.) the whole value is rendered.

Type dispatch and template construction are driven entirely from Ash.Info.Manifest.generate/1 IR — no Ash.Resource.Info.* traversal here.