docs(adr): add ADR-0001 separating additionalProperties from x-gts-sealed

- Document three concrete problems with the current spec's overloaded use of
  additionalProperties: false — allOf-applicator footguns, forward-compatibility
  break, and missing middle ground between fully-open derivation and x-gts-final
- Connect the closed content model to the tolerant reader principle (Postel's
  law), with pipeline-enrichment, vendor-extension, and producer-side-optionality
  scenarios beyond pure version evolution
- Propose x-gts-sealed as a per-schema-object derivation-control modifier
  enforced by OP#12; demote additionalProperties: false to its native JSON
  Schema instance-validation role
- Capture considered options with explicit per-option mapping of which problems
  they address and worked code demonstrations, plus impact on OP#6/OP#12 and
  reference implementations
- Place the ADR at repo-root adr/ for visibility

Author: aviator5