Add multi-database routing for `transaction.atomic(...)` and runtime-bound ORM access

[danceroutine]

Summary

Tango's current runtime model is centered on one selected environment and one default runtime/database target at a time. That keeps the common case straightforward, but it becomes a real limitation once transaction work needs to route across multiple databases intentionally.

Current codebase context

• packages/orm/src/runtime/defaultRuntime.ts exposes a singleton default runtime.
• packages/orm/src/transaction/atomic.ts always binds to getTangoRuntime().
• packages/orm/src/manager/registerModelObjects.ts and runtime-bound managers assume the default runtime path.
• Config today models one db block per environment rather than a routing story across multiple targets.

Problem

The core transaction API now exists, but it cannot yet express routing decisions such as:

• run this transaction against a non-default database
• route one model family to one database and another family to a different database
• ensure runtime-bound managers and transaction.atomic(...) agree on the same routed client

Without a routing contract, multi-database support stays outside the supported transaction story.

Proposed implementation plan

1. Define the routing contract.
   • Decide whether routing is explicit per call, model-based, request-scoped, or a combination.
   • Keep the first version narrow enough that transaction semantics stay understandable.
2. Generalize runtime/database resolution.
   • Teach the runtime layer to resolve more than one database target intentionally.
   • Ensure transaction engines are keyed by the effective routed client/runtime rather than assuming one global default.
3. Thread routing through ORM access points.
   • Align transaction.atomic(...), manager-backed writes, and runtime-bound query execution so they can share the same routed client.
   • Fail clearly when incompatible routing contexts are mixed.
4. Update config/docs/tests.
   • Define how routing is configured or injected.
   • Cover nested transactions, default fallbacks, and misrouting cases in tests.

Acceptance criteria

• Tango has a documented routing contract for transaction work against multiple database targets.
• transaction.atomic(...) can target something other than the implicit default runtime path.
• Runtime-bound managers and queryset execution can participate in the routed transaction story coherently.
• Mixed or incompatible routing contexts fail with clear errors.
• Tests cover routing behavior, nested transactions, and default fallback cases.