Add GitHub Actions CI matrix to catch toolchain regressions

[mairas]

Problem

ReactESP currently has no automated CI. The repo contains only a stub .travis.yml (entirely commented-out template). All toolchain coverage is implicit, performed manually by the maintainer against whatever PlatformIO environment is convenient.

This is the direct root cause of the 3.3.0 → C++14 regression fixed in #40:

• Replace priority_queue with std::set for immediate timed event removal #38 introduced std::set::extract() / insert(node_type) in the hot path — a C++17 construct.
• The change built cleanly on the maintainer's pioarduino setup (ESP-IDF 5.x, gnu++2a).
• It silently broke espressif32 @ ^6.x (ESP-IDF 4.4.x, C++14), which is what SensESP and other downstreams build against.
• No CI matrix existed to catch the divergence, so 3.3.0 shipped broken on a major downstream toolchain.

The C++14 fallback added in #40 restores compilation but is equally unguarded against the next perf change that accidentally reaches for a newer-standard feature.

Proposed scope

Add a GitHub Actions workflow that builds both examples/minimal and examples/torture_test across, at minimum:

• Platforms: arduino (espressif32 ^6.x — exercises the C++14 fallback) and pioarduino (newer, exercises the C++17 hot path).
• Boards: esp32 and esp32c3.

Trigger on PRs and pushes to master. Failure should block merge.

Out of scope

• Unit/integration tests on real hardware. CI-time compile coverage is sufficient to catch the class of regression this issue is about; runtime testing is a larger separate effort.
• Replacing .travis.yml. The stub can stay or be deleted in a follow-up; not blocking.

Related

• Replace priority_queue with std::set for immediate timed event removal #38 (introduced the C++17-only optimization)
• fix(event_loop): unbreak 3.3.0 on C++14 toolchains (missing <vector> + node_type) #40 (added the C++14 fallback this CI would protect)