TT1: dt policy (fixed timestep vs admitted dt stream)

[flyingrobots]

From docs/spec-time-streams-and-wormholes.md: clarify whether simulation dt is fixed (Chronos tick implies fixed delta) or whether dt is itself an admitted stream.

Scope:

• Decide on default policy for Echo (likely fixed timestep), and define the rule for allowing variable dt (if allowed) as a TimeStream + StreamAdmissionDecision.
• Specify how dt interacts with determinism, replay, and catch-up predicates.
• Update relevant specs + docs.

Acceptance:

• Spec defines dt semantics in ticks/epochs, with explicit HostTime boundary and decision-record requirements if variable.