feat(greeting): let the first impression be a card, not only a paragraph

[AimTune]

The greeting could carry exactly one thing — a string, mapped to one text frame. Everything the message layer added in 0.3.0 was unreachable from it, so the one message an app is guaranteed to send was its least expressive.

What changes

A greeting now takes a string (unchanged), a described rich message, or a list mixing both — delivered in order, each landing as its own persistent frame:

greeting: (conv) => [
  `Hi ${conv.userId}! What can I do for you?`,
  mekik.messages.buttons.spec({
    buttons: [
      { label: "Track an order", value: "/track" },
      { label: "Start a return", value: "/return" },
    ],
  }),
],

Greeting = conv => new object[]
{
    $"Hi {conv.UserId}! What can I do for you?",
    Messages.ButtonsSpec([Messages.Button("Track an order", "/track"), Messages.Button("Start a return", "/return")]),
},

The one new concept: a described message

Emitting needs a node's ctx, and the greeting fires on connect, outside any run. So this adds the value form of a message — { type, data, id? } — built with mekik.messageSpec / Messages.Spec, or typed per kind: messages.card.spec(data) and Messages.CardSpec(…), the same parameters as the emitter minus the ctx. In .NET the emitters are now one-liners over their own …Spec builders, so each type keeps one parameter list rather than two.

GenUI components are deliberately not accepted there: a chunk belongs to a turn's stream, and on connect there is no stream to attach it to. That's called out where a reader would look for it (messages page, getting-started, PROTOCOL.md §4.5).

Safety and compatibility

• Reserved frame types are refused by the builders (they throw) and dropped by the engine if a hand-built spec names one — the same rule the mapper applies, so a greeting can't collide with the protocol's own frames.
• Empty strings still greet nothing.
• Source-compatible: greeting: () => "hi" still typechecks in both languages (.NET's Func<…, string?> is assignable to Func<…, object?> by covariance).
• Wire-compatible: no new frame type, no PROTOCOL_VERSION change — the greeting simply emits frames that already existed.

Verification

Unit tests for the spec builders plus engine scenarios in both languages: ordering, the spec's id reaching the wire, gap-free seq across greeting frames, replay-not-re-greet on reconnect, the empty-string skip, and the reserved-type drop. pnpm check green (53 TS tests + every example probe); dotnet test 49/49; Docusaurus build clean.

examples/storefront.ts now greets with prose plus a button message and asserts both frames arrive before any turn.

🤖 Generated with Claude Code