gate-2: characteristic theorem family audit, revision 4

Audit the Gate 2 nominees against the falsifier in
roadmap-gates.adoc - naturally echo-shaped, not reducible to
generic sigma or fiber lemmas. Verdict: 4-of-4 surviving.

Roadmap amendments:
  Withdraw EchoCharacteristic.visible-constraint, reduces to
    proj_2; retained as expository.
  Withdraw EchoIntegration.knowledge-and-controlled-degradation,
    a literal product of two facts over disjoint frameworks.
  Nominate EchoGraded.degrade-via-join, categorical join law.
  Name EchoGraded.degrade-compose explicitly.
  Add EchoChoreo.applyChoreo-compose, role-reachability transport.
  Add EchoLinear.degradeMode-compose, linearity-mode weakening.

New construction closes EI-1:
  RoleGraded introduces RoleGEcho : Role -> Grade -> Set with
  two independent actions and a commuting-square theorem
  choreo-grade-commute. Both decoration witnesses appear on both
  sides; satisfies the same-data test.

Honest qualification: of choreo-grade-commute 18 cases,
exactly one carries non-trivial content. EI-1 closure is
protocol-correct but substantively narrow. The follow-up
question is tracked as EI-2 and closed in commit 7.

Author: hyperpolymath