feat(test-emu): add ESP32-C3 RISC-V QEMU support

[zackees]

Summary

Extends fbuild test-emu --emulator qemu (and fbuild deploy --to emu) to
cover the RISC-V ESP32 family in addition to the existing Xtensa support.
This unblocks FastLED's CI migration off Docker onto the native
fbuild test-emu runner for ESP32-C3 parts.

• Xtensa (qemu-system-xtensa): esp32, esp32s3 (existing, unchanged)
• RISC-V (qemu-system-riscv32): esp32c3, esp32c6, esp32h2 (new)

Closes #85.

What changed

• fbuild-packages::toolchain::esp_qemu grew a shared EspQemuArch enum
  and a generic EspQemu package handle. The existing EspQemuXtensa
  wrapper keeps its public API; a new EspQemuRiscv32 wrapper ships the
  RISC-V variant. Same upstream release tag (esp-develop-9.2.2-20250817),
  archive stem differs (qemu-riscv32-softmmu-...). SHA256s pulled from
  the upstream release for all five host targets (Windows mingw, Linux
  x86_64/aarch64, macOS x86_64/aarch64). New FBUILD_QEMU_RISCV32_PATH
  env override mirrors the Xtensa one.
• fbuild-daemon::handlers::emulator:
  • is_qemu_supported_esp32_mcu now delegates to EspQemuArch::for_mcu,
    which accepts C3/C6/H2 and rejects S2/P4.
  • New resolve_esp_qemu_for_mcu helper picks the right binary based on
    MCU and replaces the previous hard-coded EspQemuXtensa::new calls
    in deploy_qemu and QemuRunner::run.
  • select_runner error messages updated to list both arches.
• fbuild-deploy::esp32::build_qemu_args needed no functional change —
  it already emits -machine <mcu> / driver=timer.<mcu>.timg, which
  matches what qemu-system-riscv32 expects for C3/C6/H2. The doc comment
  was updated to reflect the expanded MCU set. PSRAM branch is a no-op on
  RISC-V parts.
• README emulator support matrix, auto-detection rules, QEMU notes, and a
  sample [env:esp32c3] block updated.

Test plan

• cargo build --release in fbuild root builds cleanly
• cargo test --release -p fbuild-packages --lib esp_qemu — 7 passing
  (new: arch_for_mcu_maps_xtensa_and_riscv,
  find_qemu_binary_direct_bin_riscv, plus existing xtensa variants)
• cargo test --release -p fbuild-daemon --lib handlers::emulator —
  32 passing, 0 failures including:
  • select_runner_explicit_qemu_accepts_esp32c3
  • select_runner_explicit_qemu_accepts_esp32c6
  • select_runner_explicit_qemu_accepts_esp32h2
  • select_runner_explicit_qemu_rejects_esp32s2
  • is_qemu_supported_esp32_mcu_accepts_riscv
  • is_qemu_supported_esp32_mcu_rejects_unsupported (s2, p4, atmega328p)
• End-to-end smoke: built an examples/Blink sketch for esp32c3 in
  FastLED via fbuild build -e esp32c3 (150.8s cold), then invoked
  fbuild test-emu --emulator qemu --environment esp32c3. The
  qemu-system-riscv32 bundle was auto-downloaded, the C3 firmware
  booted into the arduino-esp32 bootloader + app, hit setup()/loop(),
  and streamed serial output. No "unsupported MCU" error. The RMT
  peripheral subsequently timed out inside showPixels() — that's
  expected under QEMU (RMT is not fully modeled) and outside the
  scope of this PR, which is purely about making the runner
  reachable for C3/C6/H2.
• CI (Windows / Linux / macOS runners) — pending this PR

Confidence

• C3 runner wired end-to-end and verified locally on Windows x86_64.
• C6 / H2 added for free since they share the RISC-V path. Unit tests
  exercise select_runner and package URL synthesis; a hardware-equivalent
  smoke run was only done for C3.
• Cross-host: SHA256s taken from the upstream release manifest. The macOS
  and Linux bundles are downloaded on-demand; fbuild will refuse the
  install if the SHA doesn't match.

🤖 Generated with Claude Code

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📥 Commits

Reviewing files that changed from the base of the PR and between a50d906 and 9d32b23.

📒 Files selected for processing (5)

• README.md
• crates/fbuild-daemon/src/handlers/emulator.rs
• crates/fbuild-deploy/src/esp32.rs
• crates/fbuild-packages/src/toolchain/esp_qemu.rs
• crates/fbuild-packages/src/toolchain/mod.rs

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🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/qemu-esp32c3-riscv

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