find-api: discover APIs in a cached latest-stable Windows App SDK catalog

@Jaylyn-Barbee is already working on this.

Since Sep 14, 2026.

Split out from review feedback on #744, which asked for this as a follow-up rather than part of that PR. `find-api` answers only from what a project already references, which is correct for "does my code compile against this" but leaves a real gap: a member that exists in a newer SDK reads as simply absent.

From the review:

> This is a feature addition, not a defect in local-only lookup. An older project already has `LanguageModel` but lacks `GenerateStructuredJsonResponseAsync`; a newer SDK contains it.
>
> Please allow discovery in a cached latest-stable Windows App SDK catalog, clearly separate from project references:
>
> ```text
> GenerateStructuredJsonResponseAsync
> Project index (AI 1.8.53): no matching member
> SDK catalog (Windows App SDK 2.4.0 / AI 2.4.4): found
> ```
>
> The experiment acquired about **175 MB** of selected SDK/API packages and produced a roughly **4.9 MB** metadata cache. Make the initial download explicit and reuse the cache; do not modify the project or install a runtime. "Found in version X" must not imply a minimum version, compilation success, or device compatibility.

## What this needs to get right

- **Separate from project references.** The catalog answer is a second section, never merged into the project index result. A reader must always be able to tell which one answered.
- **Explicit first download.** ~175 MB is not something to acquire silently behind a search. Ask, or require an opt-in verb.
- **Reuse the cache.** The ~4.9 MB metadata cache is the artifact that matters; the packages it was built from should not need to stay on disk.
- **No project mutation.** No package added, no restore triggered, no runtime installed.
- **Careful wording.** "Found in Windows App SDK 2.4.0" states where it was found. It must not be read as a minimum version, a guarantee the call compiles, or a claim about device compatibility.

## Not in scope

Recognizing terms that appear nowhere in package information (for example "Phi Silica" or "LLM" when the metadata never uses them). An honest miss is better than an unrelated match.