flake(test): Windows emulator tests spawn 'powershell' by basename, fail under concurrent load

[zackees]

Symptom

Under uv run cargo test --workspace on Windows, handlers::emulator::tests::* tests that use test_process_command() occasionally fail with:

```
thread 'handlers::emulator::tests::run_avr8js_headless_captures_stdout' panicked at crates\fbuild-daemon\src\handlers\emulator.rs:2523:10:
called `Result::unwrap()` on an `Err` value: DeployFailed("failed to launch QEMU at powershell: program not found")
```

• Observed at least twice this session (different tests each time: once is_qemu_supported_esp32_mcu_rejects_unsupported, once run_avr8js_headless_captures_stdout).
• Always passes when re-run in isolation or with a smaller test subset.
• Specifically triggers under full-workspace concurrent load where many tests spawn subprocesses in parallel.

Root cause

crates/fbuild-daemon/src/handlers/emulator.rs:2291 — test_process_command() returns PathBuf::from(\"powershell\") on Windows, a bare basename. Command::spawn() then relies on Windows executable-search semantics (SearchPathW plus Rust's own resolution wrapper) to find powershell.exe. Under concurrent load — many tests creating/deleting tempdirs, changing process state — this resolution occasionally fails and CreateProcessW returns ERROR_FILE_NOT_FOUND ("program not found").

Fix

Use an absolute path to PowerShell in the test harness. %SystemRoot%\System32\WindowsPowerShell\v1.0\powershell.exe exists on every Windows install since Vista and is not subject to PATH races.

```rust
#[cfg(windows)]
{
let system_root = std::env::var("SystemRoot").unwrap_or_else(|_| "C:\Windows".to_string());
let exe = PathBuf::from(system_root).join(r"System32\WindowsPowerShell\v1.0\powershell.exe");
(exe, vec![ /* unchanged args */ ])
}
```

Acceptance

• Run uv run cargo test --workspace 5× consecutively on Windows without flakes in handlers::emulator::tests.
• Test still passes on a stripped-PATH environment.

Related

• perf(build): investigate warm-pass compilation stall — 30s where cache says <1s #91 — warm-build profiling uncovered Windows handle-inheritance issues in Command::spawn; this is the same class of problem (Rust std::process::Command has Windows-specific footguns).