docs: add material map, desktop-icon, and pinned-embed sections

[IamMrCupp]

Closes #33. Closes #34. Closes #35. Part of #26 (Phase 3 — extension).

Documents how to consume the new Phase 3 assets, plus the physical-brand gap the audit turned up. Prose only — all outside the <!-- GENERATED --> blocks, so make check stays green.

• BRANDING.md → Physical materials (C.2 — material / filament ↔ palette mapping (doc) #33): the screen-color → filament/anodize mapping — black body (ink), teal accent, amber for safety — plus the rule that the full spectrum is for light, not surfaces. Includes a The mark in CAD note pointing hardware at audiophore-symbol-solid.svg with the 320-unit → mm scale.
• BRANDING.md → Desktop apps (Tauri) (D.2 — desktop-app icon consumption section (doc) #35): maps the generated logos/icons/ files to the tauri.conf.json icon array, and notes the web favicon.ico isn't a substitute for icon.ico.
• README → Consuming the kit (D.1 — tag-pinned embed recipe (doc) #34): a tag-pinned raw/CDN image-URL recipe for HTTP embeds — pin to a tag, point at this repo, don't fork the SVG. Closes the .github anti-pattern (a main-pinned forked copy); retiring that forked SVG is a follow-up in the .github repo.