Custom nodes hold their inputs strongly while up-to-date

[krauthaufen]

Fixes #122.

Dependency edges are weak (Outputs only), so a node that is created and read inside a custom compute function is unreachable once evaluation returns. After the next GC the chain source -> inner -> outer is gone and outer is never marked again — silently.

Fix, the idiomatic one: a custom node holds the inputs it read strongly for as long as it is up-to-date, exactly like bind holds its inner node. New public interface IInputHoldingObject with AddInput; EvaluateAlways hands each read input to the caller if it implements it (one type test next to the existing Outputs.Add; nothing for other objects, no IAdaptiveObject change). AVal.custom and the ASet/AMap/AList.custom readers implement it: clear on Mark, refill during Compute. No locking needed — AddInput runs under the caller's monitor by construction, Mark under the transaction's EnterWrite.

Test: all seven shapes from the issue plus reads inside the collection customs, forcing a full GC between reads; fails without the EvaluateAlways call site, passes with it. Full suite green.