fix(rufield-viewer): correct ApiFieldPayload wire schema for live mode

[fallen-pc]

Summary

rufield-viewer --source live --upstream <url> never actually worked against a real RuView (wifi-densepose-sensing-server) upstream — the dashboard showed DISCONNECTED forever, even though every HTTP request to /api/field was succeeding at the network level.

Root cause: ApiFieldPayload (in crates/rufield-viewer/src/live.rs) doesn't match RuView's actual /api/field response shape (rufield_surface::api_field in wifi-densepose-sensing-server):

• dev_signing_key is a JSON bool on RuView's wire ("dev_signing_key": s.using_dev_key()), but this crate declared it Option<String>. A type mismatch on decode fails the whole payload — serde_json::from_str errors before ever touching events — so the ring's events were never parsed, on every single poll. This was the fatal one.
• signer_pubkey doesn't exist on the wire; RuView sends signer_pubkey_hex. This one was silently absorbed by #[serde(default)] (it's an informational field, never read downstream after decode), so it only caused quiet data loss, not a decode failure — but worth fixing alongside the fatal one since we're already here.

Change

• dev_signing_key: Option<String> → Option<bool>
• signer_pubkey: Option<String> → #[serde(rename = "signer_pubkey_hex")] signer_pubkey: Option<String>
• Updated the one existing test (api_payload_deserializes_verifies_and_renders) that had encoded the old, wrong shape — it now doubles as the regression test for this exact bug.

Test plan

• cargo test -p rufield-viewer — 26/26 passing (no regressions)
• Live-verified against a real RuView instance: ran wifi-densepose-sensing-server --source simulated with a real WDP_RUFIELD_SIGNING_SEED (ed25519-signed, non-dev key), pointed this crate's rufield-viewer at it.
  • Before the fix: every poll failed with invalid type: boolean false, expected a string at line 1 column 24; /api/source reported DISCONNECTED indefinitely.
  • After the fix: /api/source reports LIVE, and /events streams real events with "verified":true,"fusable":true" — RuCelium's own independent receipt verification passing against RuView's live signed output.