Add catalog discovery (browse, crawl) and deepen the records

[tamnd]

This adds two ways to discover the catalog and fills the gaps in what each record captures, after a deep audit of every data model, field, page, and piece of metadata against the live endpoints.

Discovery: browse and crawl

st browse reads the same paginated search/results list the storefront calls as it scrolls, so it enumerates the whole catalog (a total around 260,000 apps) rather than the ten-hit cap of storesearch.
Each row is a full app reference, so a hit feeds st app, st reviews, or st crawl.
Filters: --query, --sort, --maxprice, --start.

st crawl walks the typed kit:link edges breadth-first from an app, package, or profile.
An app reaches its DLC, demos, base game, and packages; a package reaches its apps; a profile reaches its most-played apps.
It emits one CrawlNode per visited record (keyed kind:id), never revisits a node, and is best-effort past the seed.
It uses a lightweight appCore read so following an app's edges does not pay for the store-page and review-summary enrichment.
Flag: --depth.

A deeper app record

st app now captures the appdetails fields earlier versions read past:

• Per-board content ratings (ESRB, PEGI, USK, ...).
• Per-platform system requirements (pc_requirements, mac_requirements, linux_requirements), parsed leniently since the store returns an object when supported and an empty array when not.
• Buy options from package_groups, with any package they name folded into the edges.
• Highlighted achievements with their localized names and icon URLs.
• Extra store art (capsule_image, capsule_imagev5, background_raw) and the legal, DRM, and external-account notices.
• Demos, as records and crawlable edges.

It also folds in the canonical review summary from the appreviews query_summary: review_score_desc (the "Overwhelmingly Positive" label), total_reviews, total_positive, total_negative.

A fixed package record

st package read page_content and header_image, keys the endpoint does not return, so its description and image were always empty.
It now reads page_image, small_logo, the controller support, and the individual price.

Smaller additions

st search carries the controller support a hit reports, and st news carries each item's feed_type.

Upgrade notes

The package record dropped the always-empty description and header_image fields and replaced them with page_image and small_logo.

Tests and gate

New tests cover the new app fields, the package fix, the catalog row parser (including the whitespace-wrapped responsive_secondrow release date), and the crawl breadth-first walk.
gofmt, go vet, go test ./..., golangci-lint, and CGO_ENABLED=0 go build all pass.