Move SWIM settings under spec.membership.swim, so Local networks carry none #488

Problem

GridNetwork.spec.swim (gossipNodes, probeInterval, suspicionTimeout) and spec.seeds are top-level fields with schema defaults, so the API server writes them into every GridNetwork, including networks that declare membership.mode: Local. A Local GridNetwork therefore carries SWIM settings that do not apply to it.

• spec.swim is never read. SwimConfig and its defaults are defined in crates/operator/src/crd/grid_network.rs, and outside that file's tests nothing references it.
• spec.seeds is read, but only to join SWIM.

Observed on both lab members on v0.3.0-rc.2: each Local GridNetwork carries the defaulted swim block and seeds: [].

Proposal

Move the SWIM settings under the membership mode that uses them, as a discriminated union:

spec:
  membership:
    mode: Swim
    swim:
      seeds: ["east.example:7946"]
      gossipNodes: 3
      probeInterval: 5s
      suspicionTimeout: 10s
---
spec:
  membership:
    mode: Local

• spec.membership.swim is optional, with no schema default.
• A CEL rule allows membership.swim only when mode is Swim, alongside the existing rule that Local requires poll.
• Remove top-level spec.swim and spec.seeds.
• Wire gossipNodes, probeInterval and suspicionTimeout into the SWIM runtime, or leave them out of membership.swim if nothing should read them.
• Breaking for v0.3.0. Document the move in the 0.3.x migration guide: a Swim network moves its seeds and swim values under membership.swim; a Local network drops them.

Acceptance criteria

• A Local GridNetwork has no SWIM fields after admission, and the API refuses one that declares membership.swim.
• A Swim GridNetwork's membership.swim.seeds are announced to the SWIM runtime as spec.seeds is today.
• Any SWIM tuning kept under membership.swim reaches the runtime, with a test.
• docs/architecture/crds.md and the migration guide match the new shape.