docs(spec): add spec 048 — lazy, trimmable control registration (#466)

Follow-on to spec 047. Settles how V1 control registration should happen so
it is automatic, lazy, and — the load-bearing requirement — does not defeat
AOT trimming. Captures the discussion and resolution:

- The pick-two-of-three tension (pure-data elements + automatic registration +
  trimmability can't coexist), grounded in the trimmer following static refs.
- Rejected approaches (per-host RegisterHandler, element self-description/cctor,
  [ModuleInitializer], reflection discovery) each tied to the requirement they
  violate.
- Resolution: factory-as-registration-link, with Pattern A (static-cctor factory
  holder for 3P controls) and Pattern B (generic Reg<E,C,H> scale pattern for the
  built-in catalog).
- ControlRegistry contract, dispatch precedence, hot-path cost model, built-in
  migration (dismantle RegisterV1BuiltInHandlers, keep Factories aggregation-free),
  trim verification test, open questions, and phasing.

Design only — no implementation.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: codemonkeychris