* docs(readme): replace tier-cascade ASCII diagram with animated SMIL SVG
The 4-tier auto-fallback block in the README becomes a self-contained
animated SVG (docs/diagrams/tier-cascade.svg, 16 KB): a 16s loop in 4
acts where requests flow from the IDE through the smart router into the
active tier, and each quota-out/budget-hit transition hands the traffic
down to the next tier, ending on the always-on free tier. SMIL only — no
JS, no external fonts — so it animates inside GitHub's camo/<img>
sandbox. Content is verbatim from the previous ASCII art; the full flow
is preserved in the img alt text. docs/diagrams/README.md gains a
hand-authored-diagrams section documenting it.
* docs(changelog): add fragment for #7615 (animated tier-cascade SVG)
* docs(readme): align tier-cascade SVG palette with DESIGN_SYSTEM.md
Retrofit to the canonical tokens (docs/architecture/DESIGN_SYSTEM.md §3.1):
dark bg #0b0e14 + the 32px graph-paper grid wallpaper (the product/site
signature), surface #161b22, borders rgba(255,255,255,.08), radius 14,
text-muted #a1a1aa. Brand semantics fixed: the router hub glyph + glow now
use primary #e54d5e (matching the favicon hub mark) and the title carries
the --grad-brand gradient (primary → accent-3); exhaustion states
(quota out / budget hit flashes, spent-tier status dots, hand-off dots)
move from brand coral to the semantic error token #ef4444; topology paths
use accent #6366f1 with accent-2 #8b5cf6 request dots; success stays
#22c55e. Re-validated (0 warnings) and re-verified frame-by-frame.
