Reconcile's withdrawal sent an untyped error with no subscription id and left the
peer holding the rows the subscription had delivered. It now sends removals for
those rows, then a variant_unsupported error whose details carry
field "subscriptionId" and the id (the existing details slots; no new wire key).
The subscribe and per-change paths use the same code. The error schema and the
Rust CultNetErrorCode accept variant_unsupported, so a Rust peer decodes it
instead of refusing an unknown code.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
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