style: standardise NatSpec on the block comment form

[mfw78]

The tree mixed /// and /** */ NatSpec, and in sixteen files both forms appeared side by side. Standardise on the block form:

• 57 declaration-level /// runs converted to /** */ across src, test and script
• block form was already the majority (85 of 142 declaration-level doc comments) and is what the newer interface and handler work in this stack already uses
• records the rule in docs/style.md so it does not drift again

Deliberately out of scope: comments inside a function body. solc ignores NatSpec tags there, so those are implementation comments rather than documentation, and expanding a one-line note into a three-line block would make the code harder to read. There are 16 of them and they are handled separately at the tip of the stack, so that PR can be dropped independently.

One case needed care: a solhint-disable-next-line directive sat in the middle of a /// run in GPv2TradeEncoder.sol. Converting naively split the header into two blocks, which silently orphans the leading one, because only the block immediately preceding a declaration is NatSpec. The halves are rejoined and the directive dropped (solhint is not configured or run anywhere in this repo).

How to test

The change is comment-only, which is machine-checkable:

git diff chore/solc-0830-toolchain..style/natspec-block-form -U0 \
  | grep '^[+-]' | grep -v '^\(+++\|---\)' \
  | grep -v '^[+-][[:space:]]*\(///\|/\*\*\|\*/\|\*\)'

That prints nothing: no non-comment line is touched. forge build and forge test are green (74 passing).

Part of #1.