fix(spruce): Prevent OOM during ROM cache population

[boburning]

What changed

• reduce ROM cache API pages from 1,000 items to 250
• persist each page before fetching the next instead of retaining every ROM for a platform
• add a regression test proving that each page is committed before the next request

Why

fetchPlatformGames previously preallocated a slice for a platform's complete ROM count and appended every API page before writing to SQLite. Responses include nested file and metadata payloads, so large libraries can exhaust 128 MB handhelds during the initial background cache sync.

On a Miyoo Mini Plus running spruceOS, the official ARM32 build exited after 23 seconds while fetching a 1,314-ROM FinalBurn Neo platform. Grout reached about 65 MB RSS immediately before disappearing; the device has about 100 MB usable RAM and no swap.

Page-sized transactions bound the retained response and database batching working set while preserving full and incremental update behavior. SavePlatformGames already replaces rows per game, so committing individual pages is compatible with both paths.

Device validation

Built the patched ARMv7 binary with the repository's docker/32.Dockerfile and tested it on the same Miyoo Mini Plus with swap disabled.

• completed a full cache population for 23 mapped platforms
• handled 1,457 NES, 1,314 FinalBurn Neo, 1,289 SNES, 1,181 GBA, 1,087 PlayStation, and 1,006 Genesis ROMs
• completed collection and 3,042 collection-mapping writes
• emitted Cache population completed and BackgroundSync: Sync completed
• completed a subsequent incremental refresh using the recorded refresh timestamps

Checks

• go test ./cache
• gofmt on changed Go files
• git diff --check
• ARMv7 Docker build using docker/32.Dockerfile