[2f] Add TypeScript declarations, JSDoc, and fix nftMarkerCount/markerNFTCount naming

[kalwalt]

Summary

Add TypeScript declarations, JSDoc on all public methods, and resolve nftMarkerCount / markerNFTCount naming inconsistency

Environment

• Product/Service: FeatureSET-Display — JavaScript API
• Files: src/ARFset.js and related source files

Problem Description

Three related API-quality gaps:

1. No type declarations: No .d.ts file exists, so TypeScript consumers get no type checking or IDE autocompletion.
2. No JSDoc: Public methods have no documentation, making the API opaque to new consumers.
3. Naming inconsistency: nftMarkerCount (field name) and markerNFTCount (local variable name) diverge — one of them is wrong, and the inconsistency is a footgun for callers.

Expected Behavior

• A .d.ts declaration file (or TypeScript source) covers all public API surface.
• Every public method has a JSDoc comment with parameter types, return type, and a one-line description.
• nftMarkerCount / markerNFTCount are reconciled to a single consistent name throughout the codebase.

Actual Behavior

No type declarations, no JSDoc, inconsistent public naming.

Tasks

• Add TypeScript declarations (.d.ts) — or convert src/ to TypeScript outright
• Add JSDoc on every public method
• Decide on naming: nftMarkerCount vs markerNFTCount and rename consistently throughout src/ and any examples

Impact

Low — Developer experience and API usability. No runtime behavior change.

Additional Context

Converting src/ to TypeScript directly is the higher-value option (catches bugs at compile time, generates .d.ts automatically) but is a larger scope. Adding a hand-authored .d.ts is a quick win that unblocks TypeScript consumers immediately. The naming fix should happen regardless of which approach is chosen.