[M7] Technical report scaffold filled with first results

## Why
`paper/main.tex` is a stub; expand with method + first tables.

## Scope
- Intro, method, experiments, limitations
- Cite traces/results from repo

## Acceptance criteria
- [ ] Compilable tex
- [ ] PDF artifact optional in CI

## References
- Roadmap: `docs/roadmap.md`
- Related vertical slice: `experiments/results/tool-semantic-drift-001.md`
