First XHR after goto(origin) intermittently hit "NetworkError when
attempting to fetch resource" (page net stack/anti-bot not ready),
healed only by the outer re-navigation retry (~30-45s/house in the
house_imv_backfill, #562).
_fetch_json_once now:
- settles FETCH_JSON_SETTLE_MS (default 1200, was hardcoded 500) — fewer
first-fails;
- wraps the in-page fetch() in a JS retry loop (FETCH_JSON_INPAGE_RETRIES
default 1, FETCH_JSON_RETRY_DELAY_MS default 800) that retries ONLY on
network throw, never on HTTP status (4xx/5xx short-circuit, caller
decides). An in-page retry costs ~retryDelayMs vs the ~30-45s outer
re-navigation. Last error re-thrown — outer crash-retry contract intact.
/fetch (SERP) path untouched. +2 tests (settle ms, retry params).
Refs #1917, #562
