FE-2028: Make Petrinaut's Actual Mode independent of what feeds it - #10121

🌟 Purpose

Petrinaut's Actual Mode still names its only host. ActualModeSource is { kind: "brunch"; endpoint; runId? }, the shared context carries Brunch's raw SSE events so the Events tab can export them, and the docs and AI context describe Brunch. A flow run can't be a source without pretending to be Brunch.

This PR makes the interface generic without changing how Actual Mode works. The host still supplies the net, the initial marking and the transition firings, in the same shapes. Only the source changes, the raw stream export goes, and so does the website's Brunch Actual Mode route, which nothing produces a stream for any more.

🔍 Changes

• ActualModeSource is now { id: string; label?: string }, and Petrinaut treats both as opaque. A new id starts a new timeline, and label is kept in recordings. The execution frame's sourceId is built from id.
• receivedEvents leaves the shared context, along with the raw events recording type and helper.
• The Events tab's Export Stream becomes Export Recording, and always downloads Petrinaut's recording: the net, its initial state and the firings. It used to save the raw SSE frames instead whenever the host passed them.
• The recording version stays at 1. There are no older recordings to read, so the new source shape replaces the old one in place.
• The website's /brunch route, its stream adapter in src/main/app/brunch-demo/, and the yarn brunch:fixture script are removed, with their README section and arch-docs mentions. The website's Brunch AI assistant is untouched.
• The Actual Mode docs and the AI context description are written for any host.

⚠️ Known issues

• No host in the repo provides Actual Mode until the next one is added.
• The AI assistant's Brunch UI is untouched: [EXAMPLE PR] FE-2044: Use the assistant plugin API in Brunch's chat #10105 moves Brunch's chat into its website plugin, and FE-1776: Let the assistant panel render nothing without a plugin assistant #10106 removes the built-in chat. brunchDemoMode in the user settings provider is left too, since it's dropped the next time settings are saved.
• Left for follow-up work: Actual Mode in PetrinautPreview, places inside subnet instances, coloured tokens, and the O(n²) marking replay.

🛡 Tests

• Unit tests cover the new source in recordings, in the execution frame's sourceId, and in the Events tab.
• Typecheck, lint and tests pass for @hashintel/petrinaut-core, @hashintel/petrinaut and @apps/petrinaut-website.
• Searching libs/@hashintel/petrinaut and libs/@hashintel/petrinaut-core for brunch, sse and EventSource (any case) only finds AI assistant code and docs, brunchDemoMode and its tests, and an example plugin id in a comment.