docs(design): document traits 2 refactor

[Roger-luo]

Summary

• redesign the proposed ppvm-traits-2 configuration boundary around direct type composition
• define the generalized Word and separate Indexable hashing capability
• preserve existing names such as ACMap, PauliWord, GeneralizedTableauSum, and EntryStore where their concepts remain unchanged
• document concrete Pauli, lossy, phased, and cached-hash representations
• propose a contiguous, column-major tableau representation with temporary transposition

Motivation

The current Config bundle mixes coefficient, word representation, hashing, map, and truncation choices. These design notes separate algorithm policy and storage from quantum-data representations while retaining established terminology unless an abstraction materially changes.

Impact

This PR adds design documentation only. It does not modify the current implementation or public API. The documents are intended to guide a later ppvm-traits-2 prototype and further design iteration.

Validation

• git diff --cached --check
• repository pre-commit hooks, including trailing whitespace, EOF, merge-conflict, private-key, and Hawkeye license checks

[github-actions]

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