[Fusion] Fix an unconditional nested defer being dropped when its enclosing conditional defer is disabled

[michaelstaib]

Ticket: hc-fork-2-sq8

Stacked on the TimeProvider PR. This PR's diff is against mst/fusion-executor-eviction-time-provider.

Fix: an unconditional @defer nested inside @defer(if: $d) was never delivered when d is false. When the outer defer is disabled, its selection set is inlined, but the inner deferred fragment must still arrive as its own incremental payload.

The defect was reproduced without authorization first. An incremental plan now runs when one of its delivery groups is active and the plan that owns its nearest active enclosing delivery group also runs. Previously it required its direct parent plan to run. The single activation rule shared by the executor and the evaluator is kept.

Verified at the top of this four-PR stack (0755da7, on #40): dotnet build src/All.slnx 0 errors; on net10, Fusion.Execution.Tests 1418 passed and 2 skipped, Fusion.AspNetCore.Tests 637 passed and 48 skipped, Fusion.Diagnostics.Tests 65 passed and 3 skipped, Fusion.Composition.Tests 915 passed. Other target frameworks were built but not tested.

Related defect, not fixed here and ticketed as hc-fork-2-d62: a conditional @defer(if: $d) nested inside another enabled @defer loses its fields when d is false.

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