Replace magic `_createdAt` loaded-sentinel with `Project#isLoaded` getter

[thehabes]

Context

utilities/projectReady.js checks TPEN.activeProject?._createdAt as a proxy for "the active project is fully loaded":

https://github.com/CenterForDigitalHumanities/TPEN-interfaces/blob/main/utilities/projectReady.js

if (TPEN.activeProject?._createdAt) {
  bound()
}

_createdAt is a server-populated field — its presence happens to imply the project has been fetched, but the coupling is implicit and undocumented. If the server field is ever renamed or removed, both onProjectReady and whenProjectReady silently regress to "never invoke synchronously" without any test failure.

Proposal

Project.js already tracks loaded state internally:

this.#isLoaded = true
eventDispatcher.dispatch("tpen-project-loaded", this)

Expose a public get isLoaded() getter and switch the sentinel check to:

if (TPEN.activeProject?.isLoaded) {
  // ...
}

Why this is better

• Removes the magic / undocumented dependency on a server field name.
• The "loaded" state is owned by the thing that is loaded, not inferred from a co-occurring server property.
• Survives schema changes to the project payload.
• Self-documenting at the call site.

Scope

• Add get isLoaded() to Project.js returning #isLoaded.
• Update the two checks in utilities/projectReady.js (onProjectReady, whenProjectReady).
• Update utilities/__tests__/projectReady.test.js — replace _createdAt: Date.now() setup with isLoaded: true (or a real Project instance).

Related

• Surfaced during review of Fix Intermittent Load Error #568 (the load-race fix).
• Out of scope for Fix Intermittent Load Error #568 to keep that PR focused on the race condition.