Add protobuf source comment support to generated Rust code (#7)

## Summary

- Extract comments from protobuf `SourceCodeInfo` and emit them as Rust
doc comments (`///`) on generated structs, fields, enums, enum variants,
oneof enums, and view types
- Add `comments.rs` module that translates index-based `SourceCodeInfo`
paths into an FQN-keyed `HashMap` on `CodeGenContext`, so codegen call
sites look up comments by the proto FQN they already have
- Add `--include_source_info` to all `protoc` invocations
(`buffa-build`, WKT generation, bootstrap generation)
- Combine proto source comments with existing tag docs (e.g. `Field 1:
\`seconds\``) using a blank-line separator
- Regenerate all checked-in generated code (WKTs + bootstrap descriptor
types) with full proto documentation

## Test plan

- [x] All 1,244 existing tests pass
- [x] Clippy clean (zero warnings)
- [x] 17 new tests: 11 unit tests in `comments.rs` (edge cases: empty
SCI, nested types, whitespace-only, detached comments, empty package) +
6 integration tests in `tests/comments.rs` (end-to-end through
`generate()`)
- [x] Verified generated WKT types have full proto documentation (e.g.
`Timestamp` struct has 50+ line doc comment from proto source)
- [x] Verified bootstrap descriptor types have proto comments on fields
- [x] CI: lint-and-test, check-generated-code, conformance suite

---------

Co-authored-by: Iain McGinniss <309153+iainmcgin@users.noreply.github.com>

Author: macalinao

.github/workflows/ci.yml

@@ -128,6 +128,7 @@ jobs:
           # /usr/local/include) so the checked-in code is pinned to a
           # specific proto source, not whatever protoc version ships.
           protoc --descriptor_set_out=/tmp/wkt.pb --include_imports \
+              --include_source_info \
               -I buffa-types/protos \
               google/protobuf/any.proto \
               google/protobuf/duration.proto \

Taskfile.yml

@@ -433,7 +433,7 @@ tasks:
       Requires protoc on PATH.
     cmds:
       - >-
-        protoc --descriptor_set_out=/tmp/wkt.pb --include_imports
+        protoc --descriptor_set_out=/tmp/wkt.pb --include_imports --include_source_info
         -I buffa-types/protos
         google/protobuf/any.proto
         google/protobuf/duration.proto

buffa-build/src/lib.rs

@@ -519,6 +519,7 @@ fn invoke_protoc(
 
     let mut cmd = Command::new(&protoc);
     cmd.arg("--include_imports");
+    cmd.arg("--include_source_info");
     cmd.arg(format!(
         "--descriptor_set_out={}",
         descriptor_path.display()
@@ -555,6 +556,9 @@ fn invoke_protoc(
 /// module-root-relative; passing user paths to both would be a contradiction.
 /// Codegen filtering happens on our side via `files_to_generate` matching.
 fn invoke_buf() -> Result<Vec<u8>, Box<dyn std::error::Error>> {
+    // buf build includes SourceCodeInfo by default (there's an
+    // --exclude-source-info flag to disable it), so proto comments
+    // propagate to generated code without an explicit opt-in here.
     let output = Command::new("buf")
         .arg("build")
         .arg("--as-file-descriptor-set")