fix(carousel): eliminate banner "blink" on crossfade switch (#1010)

[jonfroehlich]

What & why

Fixes #1010 — the hero/banner carousel "blinks" (a pink/magenta flash) when switching between banners on multi-banner pages.

Root cause. The crossfade was symmetric: on switch, the outgoing slide faded 1→0 while the incoming faded 0→1 simultaneously. Each slide is an opaque unit (black bg + cover image), so while both are semi-transparent mid-transition, up to ~25% of #main-carousel's bright magenta gradient backdrop bleeds through at the midpoint — the visible blink.

Fix. Make the crossfade dip-free by splitting the transition (CSS only, carousel_fade.css): the incoming slide fades in over a still-opaque outgoing slide, which only snaps to 0 (0s change, delayed by the fade duration) once it's fully covered. An opaque slide always covers the backdrop → zero bleed-through, while the 0.6s crossfade look is preserved. Reduced-motion still gets an instant cut.

Context on #1010

The original 2022 report predates the Bootstrap→vanilla carousel rewrite (#1288). That rewrite already fixed the decode-lag trigger (huge undecoded images) via server-side thumbnailing; this PR addresses the crossfade backdrop-bleed that the rewrite carried over. So #1010 is now fully resolved.

Verification

Drove the real page (/project/handsight, 3 banners) in a headless browser, slowed the crossfade to 2s, and screenshotted the exact transition midpoint under both the old and new CSS:

	Midpoint mean RGB	Result
Old (symmetric)	(126, 92, 120) — blue channel spikes ~+45	magenta wash (blink)
New (asymmetric, this PR)	(120, 93, 75) — natural warm blend	clean photo-to-photo crossfade

Before / after (midpoint of the same banner switch)

Before (blink): attach issue1010_before_blink.png

After (fixed): attach issue1010_after_fixed.png

Scope / risk

• One file, transition-timing only — no markup, no JS, no contrast changes.
• prefers-reduced-motion path preserved (instant cut, no crossfade).
• Affects every carousel-fade carousel (home, project gallery, project pages) — all benefit identically.

Testing notes

I visually verified this works but not taking a video.