Discover devices automatically in the machine wizard

[knipknap]

Stacked on #371.

Summary

Adds a discovery-first flow to the machine configuration wizard. Plugging in a machine and opening "Add Machine" is now enough to get a fully configured device — no forms, no baud-rate guessing.

Architecture

Discovery is split into two processes (scan once, evaluate many):

• Scanning (machine/transport/serial_scan.py, transport layer): opens every serial port once per scan, probes it across the candidate baud rates, and captures everything the device sends — unsolicited banner first, then responses to \n / ? nudges, plus a quiet-gap drain for multi-line bursts. Firmware-agnostic; a generic mostly-printable-output heuristic decides when a port is talking.
• Evaluating (machine/driver/discovery.py): drivers declare a DISCOVERY = DeviceRecognizer(label, matches, name, firmware) class attribute — pure data, no behavior. find_all_devices() runs one scan and lets every recognizer evaluate the captured bytes. Adding serial drivers costs zero additional port opens.

Wizard flow

1. Discover page: scans while visible, lists found devices with an explicit Select button, and probes each found device automatically ($I/$$) — no interaction. Rows update live to show what the machine reported about itself (e.g. "Sculpfun iCube, 120×120 mm work area").
2. On selection, all collected identity data (USB descriptors, firmware banner, probed machine name) feeds profile matching against vendor/model tokens declared in every built-in device.yaml.
3. Unique match → the curated profile is adopted (live connection facts preserved), and every step whose data is already known is skipped — straight to Review in the common case.
4. No match → the profile picker comes up pre-filtered to matching vendors/controllers, with probe specs already merged into the working machine.

Notable details

• Grbl forks that greet with $I-style build info instead of a boot banner (e.g. Sculpfun iCube) are recognized via [VER:/[OPT:/[MSG: markers; [MSG:machine:...]/mechine: names drive both the row title and profile matching.
• A bare ok ack never claims a port by itself (Marlin acks identically) — nudge responses are accumulated so recognition has full context.
• Ports of already-found devices are excluded from rescans (avoids DTR-resetting identified machines and probe races); unplugged ports are pruned via port enumeration.
• Fixes a crash in the laser control widget when deleting the active machine (ValueError: No machine found with ID … from lazy controller resolution during signal disconnect).

Test plan

• New: tests/machine/transport/test_serial_scan.py (single-open semantics, baud ordering, garbage fall-through, exclusion, lock serialization), tests/machine/driver/test_discovery.py (one-scan-serves-all-drivers, bannerless GRBL, real Sculpfun banner end-to-end), tests/machine/device/test_profile_matching.py.
• Wizard UI tests extended: auto-probe enrichment, step-skipping routes for matched/unmatched flows, vendor-filtered picker, Select button.
• Full suite: 2199+ backend tests and 58 wizard tests green; verified against real hardware (Sculpfun iCube over USB serial).