docs: add NatSpec to StdMath and StdError

[sylvarissolutions-max]

Part of #653. Two more of the files listed in the NatSpec docs checklist.

StdMath.sol

Added `@notice`, `@param`, and `@return` tags to the library and all five internal functions:

• `abs(int256)`
• `delta(uint256, uint256)`
• `delta(int256, int256)`
• `percentDelta(uint256, uint256)`
• `percentDelta(int256, int256)`

I did not rename `INT256_MIN` to `_INT256_MIN` even though the issue mentions the underscore-prefix convention for private variables. The already-merged StdAssertions.sol work (PR #666 / StdInvariant) left its private constants without the underscore prefix (for example `FAILED_SLOT`), so I matched that pattern to keep this PR purely additive. Happy to apply the rename in a follow-up or in this PR if you prefer the underscore convention going forward. Let me know which way you want to settle it.

StdError.sol

Added `@notice` to each of the nine public panic-code constants describing what Solidity panic it corresponds to. Panic codes cross-checked against the Solidity docs.

What is not in this PR

• No logic changes
• No interface changes
• No file reordering (internal up, private down was also listed in docs: enhance documentation with NatSpec for all public functions #653 - can do that in a follow-up if you want)
• No tests touched

Testing

I do not have Foundry set up on this machine so I have not run `forge fmt --check` or `forge test` locally. The diff is 38 insertions and 0 deletions, all inside NatSpec comment blocks above existing declarations, so behavior is unchanged. CI should cover the formatting and test pass. If CI flags anything I will fix it in this branch.

Happy to split this into two PRs (one per file) if that is easier to review.

[zerosnacks]

Thanks!