Adds `Dexcord.Api.Coverage`, a testable core that diffs the compiled
endpoint route table against a pinned snapshot of Discord's official
OpenAPI spec (`priv/discord_openapi.json`). The spec is a coverage
CHECKLIST, not codegen input: it is refreshed only by the deliberate
`mix dexcord.coverage.refresh`, so upstream churn never breaks CI
spontaneously.
- `mix dexcord.coverage` compiles then exits non-zero on any in-scope
endpoint missing from the declared surface (AC1.8/AC1.9).
- `mix dexcord.coverage.refresh` pins the spec via :httpc + castore,
adding inets/ssl to the code path since Mix prunes them.
- `priv/coverage_allowlist.exs` records out-of-scope spec routes with a
comment per family; reconciled against the real pin (Social SDK
lobbies/partner-sdk, OIDC, application-by-id, scheduled-event
exceptions, join requests, thread search, monetization).
- `.gitea/workflows/ci.yml` runs the suite + gate (Actions must be
enabled on the Gitea remote).
- ratelimit_test.exs gains AC1.10 spot-checks proving the new Phase 5
route shapes collapse to bounded, id-collapsed keys.
Luna
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