docs(brand): add cosy logo symbol variants

[ajianaz]

What

Adds the official Cosy logo symbol assets (logo only, no wordmark) to docs/public/logo/: master SVG, small-size cut, black/white/brand-mono variants, and a ready-to-use PNG set (16–512 px + small-cut 16/32/48).

Why

The Cosy Fan mark was approved in the 30 Sep design session (3-tone "Ocean Sunset": blue #1864AB / cyan #22B8CF / orange #E8590C, greyscale value-tested). The repo had no canonical logo assets — this gives docs, README, and socials one source of truth. Full design rationale + guidelines live in the marketing output suite.

How

• logo-symbol.svg — master (2 outlines + 1 solid card, tilt -8°)
• logo-symbol-small.svg — filled 3-tone with white separators for ≤32px (favicon/app-icon scale)
• -black / -white / -mono-e8590c — one-colour variants (generated via export script)
• png/ — exact-size renders of both cuts
• README.md — usage rules (clear space, min size, tone order, one-colour downgrade)

Testing

• svg_audit on every SVG: 96–100/100, no warnings
• Vision QA (GLM-4.6V): tone order correct, no glitch across lockup + all mockup slides
• PNG renders verified per size (non-blank, correct dimensions)
• Asset-only diff — no code paths touched; cargo gates unaffected

Related Issues

• (none — new asset set from design session 2026-09-30)

Checklist

• Branch name follows convention (docs/)
• Branch is from develop
• Commit messages follow Conventional Commits
• No secrets or credentials committed
• One logical change per PR (no mixed concerns)

[github-actions]

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✅ No issues found. Code looks good!

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