codegen: qualify prelude names in generated service code

.changes/unreleased/Fixed-20261004-215800.yaml

@@ -0,0 +1,14 @@
+kind: Fixed
+body: |-
+    **Proto types and services named after Rust prelude items compile.**
+    Generated service code wrote `Result`, `Option`, `Box`, `Send`, `Sync`,
+    `Clone` and `Into` without a path. A message, enum or service with one of
+    those names in the same package or a parent package (`message Result`, as
+    in Arrow Flight) took its place, and the build failed inside
+    `*.__connect.rs` with errors such as E0107 (`struct takes 0 generic
+    arguments`) or E0404 (`expected trait, found struct`). Generated code now
+    writes the full path, such as `::std::result::Result`. It reaches a message
+    or service named `S` or `T`, the generic parameters of the generated `impl`
+    blocks, as `self::S`. Regenerate checked-in code to pick this up: every
+    `*.__connect.rs` file changes, and the generated API stays the same.
+time: 2026-10-04T21:58:00.000000000+00:00

CONTRIBUTING.md

@@ -108,6 +108,19 @@ Key rules:
   (`::connectrpc::...`, `::buffa::...`) — no `use` statements at module
   scope. This lets multiple generated files be `include!`d into the same
   Rust module without E0252 collisions.
+- Fully-qualified paths include **the prelude**: write
+  `::std::result::Result`, `::std::marker::Send`,
+  `::std::option::Option::Some`. In `connectrpc-build` output the stubs
+  share a module with the package's messages and enums, and in every layout
+  each `pub mod` has `use super::*;`. So `message Result` or `service Send`,
+  in the package or a parent package, shadows the prelude there. The
+  `stubs_resolve_only_their_own_names` test fails when a template writes a
+  type, trait or module that the stubs do not declare without a `::` root.
+  It skips attributes (`#[derive(Clone)]`) and primitives (`str`).
+- Every **generic type parameter** that a template declares goes in
+  `GENERIC_PARAMS`, so that a message, service or sub-package of the same
+  name is reached through `self::` (`message T` is written `self::T`). The
+  same test fails on a parameter that is not listed.
 
 ## Conformance Tests