style: use plain comments for in-body implementation notes

[mfw78]

solc only recognises NatSpec on declarations, so a @dev tag inside a function body is inert: it documents nothing and no tooling reads it. This demotes the 25 remaining cases to ordinary comments and drops the tag, so that a doc comment in the tree always means NatSpec.

• single-line notes become //
• multi-line notes become a plain /* */ block rather than a /** */ doc block

This is the companion to #42, which deliberately left in-body comments alone: expanding a one-line note inside a function body into a three-line doc block would have made the code harder to read, not easier.

This PR is separable. It sits at the tip and nothing below it depends on it, so it can be closed without affecting the rest of the train if you would rather leave in-body comments as they are.

How to test

forge build && forge test

Green (154 passing). The change touches only comments.

Part of #1.

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