fix(abi): accept runtogether form for multi-cap acronym variants (iseriser#18) (#35)

`zig_variant_candidates` previously only emitted the snake_case form.
That choked on cartridges whose hand-written Zig FFI spells multi-cap
acronyms as a single run-together word (verified on boj-server/main):

| Idris2     | Emitter said | Zig actually has |
| ---------- | ------------ | ---------------- |
| GitHub     | git_hub      | github           |
| GitLab     | git_lab      | gitlab           |
| RabbitMQ   | rabbit_mq    | rabbitmq         |
| DynamoDB   | dynamo_db    | dynamodb         |

Per iseriser#18, the runtogether form (snake with all `_` removed) is
added as an additional candidate. The verifier already accepts any
candidate, so cartridges using snake_case (`manifest_loaded`, etc.)
stay green and cartridges using runtogether (`github`, etc.) now also
pass. No cartridge files are touched.

Single-word variants (`Empty` → `empty`) do NOT get a duplicate
candidate — the runtogether form is only added when it actually
differs from the snake form.

End-to-end verified against boj-server/main:
  git-mcp:    abi-verify OK (was Class B drift)
  queues-mcp: abi-verify OK (was Class B drift)
  aws-mcp:    still drifts, but on a *different* class
              (variant-extra-in-zig: sqs_send_message, sts_*,
              not the DynamoDB normalisation case) — separate fix

8 manifest_schema unit tests pass; full suite (46 lib + 9 integration)
stays green.

Refs hyperpolymath/standards#92 (Phase 2 allowlist expansion).
Refs #18.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath