R1-02: CI pipeline v1

[aravgarg28]

R1-02: CI pipeline v1 — Completion Report

Status: complete
Branch/PR: task/R1-02-ci

What was built

.github/workflows/pr.yml — CI on every pull request and push to main (docs/build/CI_PLAN.md, R1-02 scope):

• format job: clang-format pinned to 20.1.7 via pip so formatting is reproducible across machines rather than dependent on a contributor's local LLVM version. Verified locally that 20.1.7 and the 22.x used to format the tree agree byte-for-byte.
• build matrix: linux {debug, asan-ubsan, release} + macOS debug, each configure/build/test through its preset with -Werror enabled (CI-only, per BUILD_SYSTEM.md).
• Caching: ccache (also caches the gtest/benchmark compile) + FetchContent source cache keyed on cmake/deps.cmake; per-ref concurrency with cancel-in-progress.

tidy / deps / spec-coverage / python / bench-smoke jobs are intentionally absent until their subjects exist (R1-24, R2, R1-23), so CI never references tooling not yet in the tree.

README gains a developer section (build commands, canonical clang-format version, contributing workflow, recommended branch protection); BUILD_SYSTEM.md records the pinned clang-format version.

Acceptance criteria

• pr.yml with format, linux-debug, linux-asan, linux-release, macos-debug — present
• ccache + FetchContent caching — hendrikmuhs/ccache-action + actions/cache on deps
• Branch-protection notes in a README dev section — added
• Verify: green run on a PR — see CI status on this PR

Deviations / Questions

1. clang-format pinned to 20.1.7 (not the local 22.x). Reason: reproducibility across machines; a pip-installed pinned version removes version-skew flakiness. Confirmed byte-identical output to 22.x on the current tree.
2. stagecount preset not added (see R1-01 report) — it needs pipeline stage counters that first exist in R3.

Notes for reviewer

• This is the workflow that gates every subsequent PR. Once merged to main, PRs R1-03: Core strong types #2 (R1-03) onward get automated checks.
• No benchmark numbers are produced in CI (only build+test); authoritative measurement stays on the dev Mac per METHODOLOGY.md.