Add board discovery example; make discovery fast and reliable

[mullinmax]

What

1. Adds examples/discover_boards.py, a runnable script that finds every Vector board on the network and prints information about each one (IP, name, firmware version, configured game, whether a game is live). Boards are queried concurrently; unreachable boards are reported inline. If nothing is found automatically, the script prompts for one board's IP and enumerates the rest from that board's /api/network/peers table.
2. Overhauls the library's discover() to fix unreliable single-board discovery and to return results within ~2 s instead of depending on a long listen window.

Root cause of the discovery failures

Cross-checked against the firmware's discovery service (src/common/discovery.py + the phew scheduler on warped-pinball/vector, branch claude/unicast-memory-stream):

• Boards service their discovery socket every 1.5 s, but a lone board never broadcasts FULL unprovoked — on its own it only says HELLO every 15 s. So finding a single board depended entirely on the client's one OFFLINE probe actually reaching it.
• 255.255.255.255 leaves only the default-route interface; on a multi-homed Windows machine that is often not the network the boards are on, so the probe never arrived.
• Two boards "worked" only because their mutual chatter produces FULL broadcasts the client can passively overhear.
• FULL replies are broadcast, never unicast — but a board answering a PING sends the PONG unicast to the sender's IP on port 37020, which survives networks that filter broadcasts toward the client.

Discovery changes

• Probe with PING + OFFLINE(self) every 1.5 s (matching the boards' service cadence). Either frame provokes the registry into broadcasting FULL without registering the client as a peer; PING additionally earns a unicast PONG.
• Probe from every local interface, to the limited broadcast and each subnet's /24 directed broadcast, so the right network is always reached on multi-homed machines.
• Parse HELLO and PONG replies, not just FULL. If boards are heard directly but no FULL arrives within a short settle window (0.5 s), the complete list is fetched from a heard board over unicast HTTP (/api/network/peers).
• First FULL frame still ends discovery immediately; the 20 s default timeout is now only a worst-case cap. Typical result: all boards within ~2 s.

Testing

• ruff check passes; full test suite passes (244 tests), including new tests for the PING/OFFLINE probes, directed-broadcast targets, HELLO/PONG sightings, own-probe loopback filtering, and the HTTP peer-table fallback.
• Example exercised end-to-end against faked sockets/transports for the listing, enrichment, prompt-fallback, and error paths.

🤖 Generated with Claude Code

https://claude.ai/code/session_01SFN2z3hJYtQbnRVmsJuRkd

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