Migrate package metadata to pyproject.toml with one canonical version source

[Tim55667757]

Parent Feature: #40
Parent roadmap: #28

Scope

Introduce modern build metadata after release/version semantics are documented.

Acceptance criteria

• build uses pyproject.toml;
• one canonical version source exists;
• sdist/wheel version is derivable deterministically;
• historical 1.x release provenance remains understandable;
• clean build succeeds without Travis-specific environment variables.