docs: module-level doc-comment for math.rs normalization flow (#699)

[gabrielujelistic-collab]

Explain the normalization flow in math.rs module doc

Summary

Closes #699

contracts/split/src/math.rs had a short module-level doc comment describing
why amounts are normalised, but it did not explain the normalization flow
— the three-stage pipeline that every token amount goes through. This PR
expands the //! module documentation with an explicit "Normalization flow"
section so contributors understand the contract's invariant: split math is
always performed in canonical 7-decimal units, never in native token units.

What changed

• Added a "## Normalization flow" subsection to the math.rs module doc with
  the three stages:
  1. Ingest (native scale) — amounts arrive in the token's own smallest
     unit and are never trusted for cross-token math as-is.
  2. Normalize → canonical — normalize_amount scales to the fixed
     CANONICAL_DECIMALS (7) representation; all share/ratio/fee/overflow
     arithmetic happens here.
  3. Denormalize → native — denormalize_amount reverses the scaling
     before any token::Client::transfer().
• Noted that the two helpers are exact inverses (lossless round-trip except the
  intentional integer truncation when a higher-decimal token is downscaled),
  and that negative inputs are rejected.

The existing per-function /// docs on normalize_amount / denormalize_amount
are unchanged.

Verification

Documentation-only change; no code affected. (As noted on the other split
PRs, cargo check on main currently fails for unrelated, pre-existing reasons
— unimplemented helpers/error variants from other open issues — so CI red here
is not caused by this change.)

closes #699

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