feat(rust): flat-crate proto_rust_library + prost/tonic interop fixes

[pcj]

Summary

Branch-level work for the Rust toolchain (feat/rust), plus a couple of general improvements to gencopy and root-file sync. The Rust changes converge on a flat-crate convention for proto_rust_library and fix several real-monorepo failures uncovered by running protoc-gen-prost, protoc-gen-prost-serde, and protoc-gen-tonic together.

Rust: flat-crate convention (7932580)

The consuming Starlark macro now exposes all generated types at the crate root, so the gazelle extension drops the corresponding plumbing:

• No more pkg attribute on emitted proto_rust_library rules — RustLibrary.Pkg() is retained as an internal helper for Name()/Reexports() only (pkg/rule/rules_rust/rust_library.go).
• No more _rs suffix on crate names — RustCrateName returns e.g. google_api instead of google_api_rs; the crate name is the namespace (pkg/protoc/rust_keywords.go).
• Flattened extern_path entries — transitive deps use extern_path=.{pkg}=::{crate} and the self-extern override uses extern_path=.{pkg}=crate, dropping the nested ::module::path segments (pkg/plugin/neoeinstein/prost/extern_paths.go).

Rust: three independent prost/tonic failures (1935ee5)

1. Service-only proto_libraries caused tonic to error with "Tried to insert into file that doesn't exist" because ProtocGenProstPlugin skipped libraries with no messages/enums. shouldApply and outputs now also fire on HasServices() so prost emits a stub .rs for tonic's insertion point (pkg/plugin/neoeinstein/prost/protoc-gen-prost.go).
2. Non-deterministic plugin order: BUILD attr ordering could place tonic before prost, breaking the insertion. proto_compile.bzl now sorts plugins by name inside the rule impl so CLI order is independent of attr order (rules/proto_compile.bzl).
3. Path-mismatched packages (e.g. trumid.common.auth at //trumid/common/auth/proto, grpc.health.v1 under //thirdparty/.../grpc/health/v1) failed with mv: No such file or directory. output_mappings previously only handled the Rust-keyword-escape case; generalized to emit mappings whenever RustProtocOutputDir(pkg) differs from pc.Rel (pkg/rule/rules_rust/proto_rust_library.go).

Rust: misc fixes

• WKT extern path resolution (9d81be5)
• Enumerate all protos (68cf4a6)

Root-file auto-sync (5c379a1)

After GenerateRules runs for every package, rewrite two marker-delimited regions at the repo root (no-op when markers are absent):

• Cargo.toml between # gazelle:proto_rust_members start/end — one entry per package emitting a proto_rust_library.
• BUILD.bazel between # gazelle:vendor_proto_sources_deps start/end — underlying _lib targets plus every proto_compiled_sources rule.

gencopy: ProTip with the .update target (b655336)

When bazel test //path:foo_proto_compile_test fails on drift, append a one-line ProTip to the mismatch error naming the .update target so it can be copy-pasted into bazel run:

ProTip: to regenerate, run:
    bazel run //proto:foo_proto_compile.update

Label derived from pkg.TargetLabel by swapping the rule-name component with cfg.UpdateTargetLabelName (cmd/gencopy/gencopy.go).

Test plan

• bazel test //... across the gazelle extension packages
• Generated rules build in a downstream monorepo with a mix of message-only, service-only, and path-mismatched proto packages
• protoc-gen-prost + protoc-gen-tonic + protoc-gen-prost-serde co-execute cleanly regardless of BUILD attr order
• WKT-only proto libraries resolve their extern paths correctly
• Cargo.toml / root BUILD.bazel marker regions are populated on gazelle run; left alone when markers absent
• gencopy mismatch output includes the ProTip line with the correct .update label