commitment: move deferred branch writes behind PatriciaContext

## Problem

Deferred branch updates (parallel commitment path) are managed via `BranchEncoder.CollectDeferredUpdate` and `ApplyDeferredBranchUpdates`, which call `ctx.PutBranch` directly. The deferred queue logic is interleaved with trie processing, making it hard to reason about and test.

## Proposed Solution

Extend `PatriciaContext` (or add a new `WritablePatriciaContext`) to accept deferred writes transparently:

```go
type DeferringPatriciaContext struct {
inner PatriciaContext
deferred []deferredBranchUpdate
}

func (d *DeferringPatriciaContext) PutBranch(prefix, data, prev []byte) error {
d.deferred = append(d.deferred, deferredBranchUpdate{prefix, data, prev})
return nil
}

func (d *DeferringPatriciaContext) Flush() error {
// apply all deferred updates, potentially in parallel
}
```

## Result

- Trie calls `ctx.PutBranch` uniformly — whether immediate or deferred is `PatriciaContext`'s concern
- `BranchEncoder` no longer needs deferred queue management
- Parallel and sequential paths share the same trie code
- Simpler testing: swap context implementation to control write ordering

## Related

- #20277 (move warmup cache behind PatriciaContext)