types/block.zig: evaluate or remove finishSingleRawAggregatePrep degenerate fast-path

## Context

Raised by @ch4r10t33r in [PR #925 review](https://github.com/blockblaz/zeam/pull/925#discussion_r3297617266).

`finishSingleRawAggregatePrep` in `pkgs/types/src/block.zig` is a degenerate-shape fast-path for `1 gossip sig + 0 children` that still invokes `leanMultisig` once but skips child-selection loops entirely.

## Action Required

- Measure whether this path provides a meaningful performance improvement compared to the general path
- If the improvement is negligible or unmeasurable under realistic conditions: **remove the code** and use the general path only (simpler, less surface area)
- If there is a clear improvement: document it with a benchmark and keep it

## Related

- PR #925: node, types, xmss: keep aggregate worker under 4s slot budget
- /cc @ch4r10t33r @g11tech

Start in pkgs/types/src/block.zig at finishSingleRawAggregatePrep and compare its 1 gossip sig + 0 children path with the general path. Measure both under realistic conditions, including the leanMultisig call. Done means either removing the fast-path when the improvement is negligible or retaining it with a benchmark documenting a clear improvement.

Written by the indexing model from the issue text.