fix(app): keep idle relay links alive

[frahlg]

What changed

• refresh the app relay read deadline when the relay sends a WebSocket ping
• keep Gorilla's same-payload pong reply
• add a short idle-connection regression test

Why

The relay pings every 15 seconds, while Core closes a quiet connection after 45 seconds. Core refreshed its deadline only for data frames and pong frames. A relay ping therefore did not count as activity, so a healthy idle link could time out and reconnect every 45 seconds.

Impact

Idle FTW app links stay connected while relay heartbeats arrive. Missing heartbeats still cause the same timeout and reconnect path. This does not change control or safety behavior.

Regression proof

• old code: the new 30 ms idle test timed out even while the relay sent pings every 5 ms
• new code: the focused test passed 100 runs
• new code: the focused race test passed 20 runs
• make verify passed on exact head, including 167 optimizer tests, Go tests, vet, and build
• git range-diff is identical to the reviewed local candidate

Base: b6ee4e3bf5d144da0a226fb2b0029225cc8ada6d

[frahlg]

@codex review

@miravoss26 please review exact head 4f64bdc4920cb0be88ad6f77cec5ce991ec587ae. It is patch-identical to the reviewed local candidate on post-#921 master b6ee4e3bf5d144da0a226fb2b0029225cc8ada6d. The focused test passed 100 runs, the focused race test passed 20 runs, and full make verify is green.

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Delightful!

Reviewed commit: 4f64bdc492

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