feat(spacetimedb): add the SpacetimeDB SDK

[Mkassabov]

Caution

This is a requested SDK from an Alchemy PR, not a distilled issue. There is no existing distilled PR for SpacetimeDB. The request is a review comment on alchemy-run/alchemy#1044 telling us to put the client in distilled instead of a hand-written Alchemy client.

• alchemy#1044 review: feat(spacetimedb): typed SpacetimeDB provider alchemy#1044 (comment)

This is the requested SpacetimeDB SDK.

Adds @distilled.cloud/spacetimedb. SpacetimeDB publishes no first-party OpenAPI; fetch-specs.ts snapshots the vendor HTTP docs (llms.txt + /docs/http/*) and assembles OpenAPI from that snapshot (management HTTP: /v1/database, /v1/identity, GET /v1/ping). Not a third-party spec.

• Fetch script: stacks/distilled-submodules/spec-repos/spacetimedb/fetch-specs.ts
• Docs: https://spacetimedb.com/llms.txt
• Convert/generate: packages/spacetimedb/scripts/{convert,generate}.ts
• Mirror repo distilled-mirror/spec-mirror-spacetimedb is created by the distilled-submodules stack on merge to main. Until then, regenerate with pnpm specs:local spacetimedb and DISTILLED_SPECS_LOCAL=1 pnpm generate spacetimedb.

pnpm specs:link spacetimedb recorded the .gitmodules stanza (shallow = true).