Flaky: settled-marker fixture intermittently duplicates the *-settled-fast DOM element on a HIT (both apps) #718

Pre-existing flake, base-verified during #717 work (identical commands on main): the bake-lane settled-marker test ('nested promise settled inside the window') intermittently resolves TWO elements for *-settled-fast testids on a HIT — strict-mode locator violation; one diff run failed 3/3 then 30/30 clean on rerun. Seen in both apps (test-app shell-cache + cloudflare ppr-shell.test.ts:562-region). Suspected mechanism family: the #706-disclosed $RC race (client fills a dehydrated boundary before $RC executes, orphaning fizz's hidden segment container) — functionally invisible, hidden node, zero hydration errors; one locator was already scoped in ppr-tag-eviction. Action: root-cause and fix the duplication (or scope the remaining locators with a documented invariant). Refs: #706 notes, #717 body.