Sub-unit dust is lost on release for tokens with < 8 decimals (O-5)

[dendisuhubdy]

From docs/audit/2026-09-19-reaudit.md O-5: release floors the attested amount, so a burn of 199 attested units of a 6-decimal token pays 1 native unit and strands 99 in custody, and a burn below 100 can never be released (ZeroAmount). check_burn on Rand does not know the decimals. Decide: wallet-enforced granularity, decimals in AssetInfo, or refuse a non-multiple on release (the Rand side has to enforce it first).

[dendisuhubdy]

Closing: decided and fixed on the Rand side, which is where the issue said it had to be enforced first.

• A backing now carries its source decimals, and a BridgeBurn whose amount or relayer_fee is not a whole number of the backing's release unit (10^(8 - decimals)) is refused with NotReleasable before the proof is checked (fullnode fec2947, "a bridge burn must be a multiple of the release unit, so custody equals locked to the last unit").
• Pinned by a test with Ethereum USDT at its real 6 decimals: 199 and 99 wire units are refused, 200 with a fee of 100 is admitted, and locked stays a whole number of native units.
• The rule is in the builds that carried the bridge (chain 14 b3c594c, chain 15 dd2ccbe), and the live registry lists the 6-decimal backings with decimals: 6.

The endpoints are unchanged: a release still floors and still reverts ZeroAmount below one unit, but no burn that would reach either case can be made.