[pikann]

Summary

Adds two new host capabilities plugins can use: a generic escape hatch for custom host functions, and live workspace branding lookup.

• custom_host_call.go (wasip1): CallHostFunction(hostFn, request, response) — a low-level escape hatch for a plugin that needs a host capability beyond what this SDK exposes directly (e.g. an SMTP-sending mail plugin's outbound socket access), gated behind a manifest permission the plugin declares for itself. The plugin author still declares their own //go:wasmimport binding using the (reqPtr, reqLen, resPtrPtr, resLenPtr int64) calling convention (CustomHostImport); CallHostFunction handles the JSON marshal/unmarshal and WASM linear-memory pointer plumbing this SDK already uses internally for Fetch/DB/etc., so plugin authors don't have to hand-roll unsafe pointer arithmetic.
• wasm_imports.go / wasm_backends.go: new paca.settings_get host import and GetBranding() wrapper. Returns BrandingInfo (logo URL, brand name, light/dark primary colors) mirroring the workspace's public GET /branding endpoint. Meant to be called at render/send time rather than cached, so a branding change is reflected immediately in anything a plugin renders afterward (e.g. an email template).
• native_backends.go: matching native (non-WASM) stubs for BrandingInfo, GetBranding, CustomHostImport, and CallHostFunction, all returning errNotWASM, so plugin code compiles and runs the same way under go test as it does under wasip1.

Deliberately not added: an email.send-style SDK function. Sending mail is specific to one class of plugin (SMTP, and later SES/SendGrid/Mailgun), not a general-purpose capability every plugin needs — a mail plugin instead declares its own private send_email go:wasmimport binding and drives it through CallHostFunction.

Test plan

• go build ./..., go vet ./... clean (native)
• GOOS=wasip1 GOARCH=wasm go build ./... clean