Add Effect.FlushlessCreateNextChild for appending numbered child effects

[stidsborg]

Summary

Adds a flushless Effect.FlushlessCreateNextChild<T> primitive that, for a given parent EffectId, finds the next free direct-child slot and records the provided content there as a completed effect.

• Next-free rule: highest existing direct-child index + 1 (append; gaps are kept).
• Returns the created child's EffectId.
• Flushless / in-memory only: the write is visible immediately (readable via Get/TryGet) and the next call already sees the reserved slot, so flushing is not needed for index correctness — durability is the caller's concern (call Effect.Flush()), matching the existing Flushless* family.
• Thread-safe: index-selection and reservation happen under a single _sync acquisition (via an extracted lock-free AddToPendingCore) so concurrent callers cannot collide on the same index.

This generalizes the hand-rolled next-child bookkeeping currently in Queuing/IdempotencyKeys.cs. Migrating IdempotencyKeys onto this primitive is intentionally left as a follow-up.

Caveat

This is a raw append: each call adds a new child, so it is not replay-idempotent on its own. It is intended for callers driven by genuinely-new external events (as IdempotencyKeys is, guarded by its own content dedup).

Tests

CreateNextChildTests (4 tests, in-memory):

• appends sequentially from 0
• continues from pre-existing children
• keeps gaps and uses highest+1 (across a persisted Clear)
• indexing is scoped per-parent