fix(docs): always emit single top-level docs.json

[raphael]

This PR standardizes the docs plugin output and resolves $ref inlining inconsistencies in multi‑package designs.

Summary

• Always emit a single aggregated gen/docs.json regardless of number of roots
• Preserve JSON tag transforms and $ref inlining across multi‑package builds
• Deterministically merge services and definitions across roots
• Remove per‑API (service‑scoped) gen/http/<API>/docs.json emission
• Update tests to reflect single‑file output

Rationale
Goa designs declare a single API. Emitting per‑API docs under gen/http/<API>/docs.json created path inconsistency and consumer confusion. It also made $ref inlining appear unreliable when scripts consumed the legacy gen/docs.json. This change restores the expected single top‑level output and keeps inlining behavior correct by scoping OpenAPI definitions per root during aggregation.

Notes

• README already documents top‑level docs.json; no changes needed.
• The output path is now stable across single‑ and multi‑package designs.
• Verified in a real multi‑package project:
  • gen/docs.json produced with zero $ref occurrences
  • No service‑scoped doc is emitted