Add EmulatorRunner abstraction and fbuild test-emu command #23

Goal

Introduce a standalone emulator execution layer and CLI command surface for post-build testing.

Why

fbuild already has build outputs and monitor semantics, but emulator execution is not implemented. The right model is build -> emulator runner -> monitor/crash classification, not routing emulation through hardware flashing logic.

Scope

• define an EmulatorRunner abstraction separate from hardware deployers
• define result types such as Passed, Failed, Crashed, TimedOut, and Unsupported
• add fbuild test-emu or equivalent command
• route emulator stdout/stderr through the existing monitor logic
• support --expect, --halt-on-success, --halt-on-error, and --timeout
• surface command line, exit code, stdout, and stderr on failure

Acceptance criteria

• emulator execution does not go through the hardware flashing path
• the CLI can build then execute a runner in one flow
• monitor semantics are reused for pass/fail classification
• unsupported boards produce explicit errors

Likely files

• crates/fbuild-cli/src/main.rs
• daemon operation models/handlers
• new emulator runner module/crate

Parent: #21