Change WakuMessage from a value object to a ref object so assignments,
async-closure captures, and Result/Option bind-outs cost a pointer +
refcount instead of deep-copying the three seqs + string.
- structural `==` (nil-aware, field-wise) so message equality assertions
keep working under ref identity
- `clone()` explicit deep copy for the rare mutate-a-held-message case
- rewrite `ensureTimestampSet` to be non-mutating (clone when unset)
instead of `result = message; result.timestamp = ...`, which under ref
semantics would mutate the shared input
- new unit tests: structural == incl. nil, clone independence,
ensureTimestampSet same-ref/fresh-ref behaviour
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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