feat(cloudflare): slim OpenFeature provider for CF resolver

[nicklasl]

Summary

• Plan for a new @spotify-confidence/openfeature-cloudflare-provider package
• Slim OpenFeature provider (~150 LOC) that talks to the CF resolver via Cloudflare Service Binding
• No WASM, no state polling, no log flushing, no client secret — the CF resolver handles all of that

What's in this PR

Just the plan (openfeature-provider/cloudflare/PLAN.md) for review. Implementation follows in subsequent commits.

Test plan

• Review plan for completeness and correctness
• Validate shared code extraction approach
• Confirm waitUntil/ctx pattern matches confidence-sdk-js

🤖 Generated with Claude Code

[nicklasl]

Future direction: Unified provider with pluggable resolver

The shared code extraction in PR 1 and the CF provider in PR 2 are stepping stones toward a single ConfidenceProvider with a pluggable Resolver interface:

interface Resolver {
  resolve(flags: string[], context: Record<string, any>): Promise<FlagBundle>;
  initialize?(): Promise<void>;
  close?(): Promise<void>;
}

Three resolver implementations, same provider:

Resolver	Backend	Use case
ServiceBindingResolver	CF Worker via service binding	Cloudflare Workers at the edge
LocalResolver	In-process WASM + CDN state polling	High-throughput servers (Node, Deno, Bun)
RemoteResolver	HTTP to resolver.confidence.dev	Simple setups, low-volume backends

What this PR already does toward that goal:

• js-shared/flag-bundle.ts extracts the shared FlagBundle type and resolve()/evaluateAssignment() — this becomes the contract between provider and resolver
• The CF provider's evaluate() method is essentially the unified provider pattern already (FlagBundle in, ResolutionDetails out)

What a follow-up would need to change:

• Refactor ConfidenceServerProviderLocal — move state polling, log flushing, materialization orchestration, and WASM lifecycle out of the provider class and into a LocalResolver implementation. This is the bulk of the work (~400 LOC moves from provider to resolver).
• Extract the provider shell — the ~50 LOC of OpenFeature glue (evaluate(), resolveBooleanEvaluation(), etc.) becomes the shared ConfidenceProvider class.
• Unify FlagBundle creation — each resolver normalizes its response format (protobuf, JSON, etc.) to FlagBundle internally, so the provider never sees protocol-specific types.
• Package consolidation — either merge into one package with entry points (/local, /cloudflare, /remote) or keep a core + resolver packages. A single package with entry points is simpler but means renaming from @spotify-confidence/openfeature-server-provider-local (breaking change). Separate resolver packages avoid the rename but add maintenance surface.