feat: wrap ResultExportConfig around use-case sub-structs

[bdchatham]

Summary

Applies the sibling-wrapper pattern from #108 to ResultExportConfig. The flat { CanonicalRPC string } becomes a wrapper around a ShadowResult *ShadowResultConfig sub-struct, which carries the canonical-RPC endpoint. Future result-export flavors slot in as siblings (scheduled, per-use-case comparisons, etc.) without renaming existing fields.

• Types: ResultExportConfig is now a wrapper; ShadowResultConfig owns canonicalRpc. Both in api/v1alpha1/common_types.go.
• Stale docs removed: the old godoc described a "scheduled mode" reachable by omitting canonicalRpc, but canonicalRpc was CRD-required (MinLength=1), so scheduled mode was never actually reachable through the CRD. Removed the prose; didn't preserve the behavior.
• Validation: replayerPlanner.Validate (the sole consumer today) rejects an empty resultExport: {} wrapper via the context-free validateResultExport helper + caller-wrap with replayer: prefix — same pattern as validateSnapshotGeneration.
• Samples: pacific-1-shadow-replayer.yaml updated to the nested shape.
• Generated: CRDs and DeepCopy regenerated via make manifests generate.
• No sidecar wiring changed: ResultExportConfig has no active consumer today (was removed with the monitor-task subsystem in feat: remove monitor task system #89); this is schema-only.

Shape

# Before
resultExport:
  canonicalRpc: "http://rpc.pacific-1.prod.platform.sei.io:26657"

# After
resultExport:
  shadowResult:
    canonicalRpc: "http://rpc.pacific-1.prod.platform.sei.io:26657"

Verification

• make build — succeeds
• make lint — 0 issues
• make test — all packages pass, including the new TestReplayerPlanner_Validate_ResultExport covering nil / valid-shadow-result / empty-wrapper-rejection

Context

Identified in the extensibility audit after #108 merged. Both the kubernetes-specialist and platform-engineer flagged ResultExportConfig as the most direct clone of the anti-pattern #108 fixed: two behaviors discriminated by presence of a string field. No launched product, no backward-compat concerns.

Test plan

• Apply pacific-1-shadow-replayer.yaml and confirm it accepts the new nested shape
• Apply a manifest with resultExport: {} (no sub-struct) and confirm the planner rejects it with a replayer: resultExport is set but has no sub-struct error
• Review the regenerated CRD YAML (manifests/sei.io_seinodes.yaml, sei.io_seinodedeployments.yaml) for the resultExport.shadowResult.canonicalRpc path
• Confirm no existing test fixtures still use the flat ResultExportConfig{ CanonicalRPC: ... } shape (grep verified clean)

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