ListAssets with group_key filter returns empty while ListBalances reports a non-zero group balance

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## Observation

Reproducible in regtest against tapd main
(\`0.7.99-alpha commit=v0.7.0-506-ge5b652e9\`) while running the
tap-sdk TestBurnAssetByGroupKey scenario:

1. Mint a grouped fungible asset of 500 units.
2. Burn 100 of them.
3. Mine 6 blocks, wait for \`synced_to_chain\`.
4. \`ListBalances --by-group --group-key \` returns
\`{balance: \"400\"}\`.
5. \`ListAssets --group-key \` (default script-key filter)
returns an empty list.

The wallet demonstrably owns 400 units of the group (the daemon's own
ledger says so) but cannot list a single asset that belongs to it. If
the script-key filter is widened to \`--all-script-keys\`, the tranche
shows up.

## Why this matters

Clients that use the semantic \`AssetRef → ListBalances → ListAssets\`
flow to enrich a balance with genesis metadata silently fall over
here. The tap-sdk has to either work around it (papering over a
daemon-level inconsistency) or propagate an error that is not true
from the user's perspective.

## Proposal

- Default \`ListAssets\` with a \`group_key\` filter should surface every
tranche whose remaining balance contributes to the \`ListBalances\`
total, regardless of script-key type. The two RPCs should not
disagree on \"what's in this group.\"
- If the current behavior is intentional (e.g. burn-output script keys
intentionally excluded), document the invariant loudly so clients
stop treating the balance as \"there's an asset somewhere.\"

## Context

Found while bringing up parity itests in tap-sdk PR #37. The SDK had a
workaround that I rolled back so the server-side issue stays visible.

Start with the tap-sdk TestBurnAssetByGroupKey scenario and trace the server-side handling of ListBalances and ListAssets for grouped assets after a burn. Reproduce the regtest sequence, compare the default script-key filter with --all-script-keys, and inspect the existing behavior before deciding whether the RPCs should align or the invariant should be documented.

Written by the indexing model from the issue text.