Misleading comment on Batch.Immutable field

[crtahlin]

The comment on the Immutable field in the Batch struct says:

https://github.com/ethersphere/bee/blob/master/pkg/postage/batch.go#L18

Immutable   bool     // if the batch allows adding new capacity (dilution)

This suggests immutability controls whether a batch can be diluted (depth increased). However, looking at the actual code:

• The increaseDepth function in the PostageStamp smart contract does not check the immutableFlag — dilution is allowed for both immutable and mutable batches.
• The Immutable flag is actually checked in stampissuer.go in the increment() method, where it controls whether chunks can be overwritten when a bucket is full. Immutable batches return ErrBucketFull, while mutable batches reset the counter and allow overwriting.

The comment should reflect the actual behavior — immutability controls whether older chunks can be overwritten when buckets fill up, not whether the batch can be diluted.

Suggested fix:

Immutable   bool     // if true, chunks cannot be overwritten when buckets are full