fix(buyer): publish discovery results incrementally

[kotevcode]

Summary

• start the buyer background discovery sweep immediately on proxy startup and periodic refresh
• emit accepted metadata results one-by-one during exhaustive DHT sweeps so buyer.state.json updates while slower endpoints continue resolving
• raise the default buyer metadata fetch timeout to 1500ms across CLI, desktop, and docs

Validation

• corepack pnpm --filter=@antseed/node build
• corepack pnpm --filter=@antseed/cli test
• corepack pnpm --filter=@antseed/desktop test (script currently fails resolving dist/main)
• cd apps/desktop && node --test dist/main/*.test.js