Backport `for_each_cacheless` to fvm2 and fvm3 state trees

[coderabbitai]

Summary

Currently, StateTree::for_each_cacheless in src/shim/state_tree.rs falls back to the caching for_each for FvmV2 and FvmV3 variants, because those upstream crates do not yet expose a cacheless iteration API. Only FvmV4 has a native for_each_cacheless.

Required Changes

1. Contribute for_each_cacheless upstream to the fvm2 and fvm3 crates (or, if upstream is not feasible, implement a workaround in the shim layer that avoids caching HAMT nodes during iteration).
2. Once available, update StateTree::for_each_cacheless in src/shim/state_tree.rs to call the native cacheless method for FvmV2 and FvmV3 variants instead of falling back to for_each.

Affected Files

• src/shim/state_tree.rs — StateTree::for_each_cacheless implementation for FvmV2 and FvmV3 arms.

Context

• Introduced in PR: fix: prefer StateTree::for_each_cacheless over StateTree::for_each #7237
• Review comment: fix: prefer StateTree::for_each_cacheless over StateTree::for_each #7237 (comment)

Acceptance Criteria

• StateTree::for_each_cacheless for FvmV2 and FvmV3 no longer caches HAMT nodes during iteration.
• Memory usage during state-tree iteration on older network versions is comparable to FvmV4.