substrateTarget routing + build validation + surfaces theme-authoring docs

Acceptance Criteria

• Surface fill target is RuneConfig.substrateTarget (default 'self', separate from frameTarget), theme-overridable via mergeThemeConfig; a per-instance substrate-target="self|media" always wins.
• Targeting media on a rune with no media section emits a build warning; the addressable media-well surface is defined for later tint reuse.
• A theme-authoring "surfaces" page documents the three fill layers (tint=colour, bg=image, substrate=pattern), the self/media-well targeting, and the Case A/B examples.