Remove the `pack!`, `pack_err!`, `pack_structural!`, and
`pack_err_structural!` macros, replacing all pipeline type definitions
with `#[derive(Grouped)]` and `#[derive(Grouped, Wrap)]` attributes.
This changes the generated struct shape from named-field structs with an
`inner` field to tuple structs accessed via `.0`, and removes the
auto-generated `name` and `info` fields from error types.

Dispatchers are now always registered at compile time, removing the
`with_dispatcher` / `with_dispatchers` methods and the
`PathfinderConfig` API. The `dispatch_tree` feature now only controls
the matching strategy (trie vs linear list).

Mark all source files with a comment indicating they are not optimized
for documentation purposes.

Restructure flat module layout into logical directories (attr/, derive/,
func/, systems/, extensions/, utils.rs). Tighten internal function
visibility to pub(crate). All public API signatures remain unchanged.