release v0.7.0: post-merge polish + archive MPS changes

[jascal]

Summary

• Bump version 0.6.0 → 0.7.0; convert CHANGELOG "Unreleased" → "0.7.0 (2026-05-01)" with corrected tier values (super-sib {0.335, 0.593, 0.753}, cross-group [0.000, 0.178]) and the cross-coupled-by-sum encoding formula in the hierarchical-polysemy entry; add Fixed entry for fix-mps-encoding-non-factorizing referencing PR fix-mps-encoding-non-factorizing: §1-§8 implementation #48.
• README: move "MPS concept encoding (rung 1)" from "Near-term — implementation pending" into "Recently shipped" with PR add-mps-concept-encoding: §1, §3-§7 — hierarchical polysemy example, demo, docs #46 / fix-mps-encoding-non-factorizing: §1-§8 implementation #48 refs.
• Research doc: link docs/research/polysemantic-encoding-beyond-product-states.md rung-1 entry to merged PRs.
• OpenSpec: archive add-mps-concept-encoding and fix-mps-encoding-non-factorizing under openspec/changes/archive/2026-05-02-…; spec deltas flow through to openspec/specs/{compiler,language}/spec.md.
• Tech debt: append §5.8–§5.15 from a post-merge self-review of PR fix-mps-encoding-non-factorizing: §1-§8 implementation #48 (HIGH: scientific-notation regression in _split_linear_combination; MEDIUM: error-message drift, etc.).

Out of scope (follow-up)

• CI smoke step that runs demos/larql_polysemantic_hierarchical/demo.py end-to-end (.github/workflows/verify-examples.yml) — split out of this PR because the local PAT lacks workflow scope. Will be added via the GitHub UI in a separate one-line edit.
• §2 of the post-merge to-do list (rung-1 utility validation: scaling sweeps n=3-8, benchmarks/mps/ scaffold, SAE-pattern validation against arXiv 2209.10652 / 2309.08600) — substantial enough to warrant its own OpenSpec change mps-rung-1-validation. To be drafted after this PR lands.

Test plan

• `pytest tests/test_examples.py -k hierarchical` — 2 passed
• `python demos/larql_polysemantic_hierarchical/demo.py` — Result: PASS
• `python -c "import q_orca; print(q_orca.version)"` → `0.7.0`
• `openspec validate --all --strict` — 5/6 pass (the tech-debt-backlog failure is a pre-existing issue: that change has no spec deltas because it's a meta tracking doc, not a feature change)
• Full pytest suite — to be re-run on CI

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