ci: replace manual protoc install with arduino/setup-protoc action

[paikend]

Summary

• Replace the repeated curl + unzip protoc installation with arduino/setup-protoc@v3 across 6 CI jobs (check, msrv-check, test, clippy, doc, wasm), gaining built-in caching across workflow runs
• Remove protoc installation from the examples job entirely — multiservice-example uses checked-in generated code and does not invoke protoc at build time
• Update the PROTOC_VERSION env comment to reflect the new setup

Motivation

As noted in #11, every CI job was independently downloading protoc on each run. arduino/setup-protoc caches the binary via @actions/tool-cache, so subsequent runs skip the download entirely.

The examples job was installing protoc unnecessarily — multiservice-example has no build.rs and include!s pre-generated code, so protoc is not required. Verified locally with cargo build -p multiservice-example without protoc installed.

Changes

Job	Before	After
check, msrv-check, test, clippy, doc, wasm	4-line curl + unzip + protoc --version	arduino/setup-protoc@v3 (2 lines)
examples	Same curl+unzip block	Removed (not needed)

Net: -38 lines, +21 lines in ci.yml.

Test plan

• YAML lint passes
• arduino/setup-protoc@v3 supports exact version pinning ("33.5") per README
• cargo build -p multiservice-example succeeds without protoc installed
• CI jobs pass on this PR

Closes #11

🤖 Generated with Claude Code