fix: eliminate ghost overlay and loading flash on story author transition

Two related issues were causing visible flicker when swiping between
story authors:

1. The fading storyContent stayed in the view tree as an opacity-0
layer through the spring's settle window (~200ms past visual
completion), so any modifier composed on top of it (overlays,
backgrounds) would render against an invisible-but-present layer
and "pop off" abruptly when the commit cleared pendingTransition.
Drop storyContent from the tree once it's fully faded with
`if swipeAmount < 1`, and mark its insertion/removal as
.transition(.identity) so SwiftUI snaps instead of fading.

2. The fullsize-loading placeholder rendered the blurred thumb via a
nested LazyImage, which takes a frame to publish even from Nuke's
memory cache — producing a 1-frame background flash on every
transition where the next story's fullsize wasn't already loaded.
Hit the cache synchronously and render via plain Image(uiImage:),
matching the pattern pendingFaceView already uses; fall back to
LazyImage only if the cache truly misses.

The cache miss itself is the underlying cause of the rare residual
flash, and stems from storyPrefetchRequests only enqueueing fullsize
URLs. Mirror feedPrefetchRequests' policy and prefetch every
reachable thumb at high priority — they're tiny and the placeholder
relies on them being cache-hot. Tests updated to use containment
assertions where exact-count assertions previously gated the high
tier.