Add release notes for everything since 1.6.1

[Jim Garrison (garrison)]

#43 adds the reno machinery but deliberately ships no notes, so the release-notes page it builds renders empty. Twenty-eight commits have landed since the 1.6.1 tag, several of which change behavior a user has to know about, and none of it is written down anywhere a user will look. This backfills that history as 19 notes: 6 upgrade, 7 features, 6 fixes.

The 1.6.1 tag is far behind main, so every commit covered here is unreleased. The notes render under "Upcoming release" until 1.7.0 is tagged, at which point reno attaches them to it — so this needs no coordination with the version bump in #42, and the notes are filed loose in releasenotes/notes/ rather than in a 1.7.0/ subdirectory.

The upgrade notes are the point of the exercise

• Multi-iteration SQD results move. The amplitudes handed back to the SQD loop were being replaced by a uniform array and are now correct (Fix wavefunction amplitudes: reshape over the full subspace, never fabricate #26). Energies and occupancies were never affected.
• h_comm_size is gone from TPB_SBD/GDB_SBD (Separate SQD parameters from SBD parameters, and expose the ones that were unreachable #28). Assignment now raises AttributeError, and — less obviously, so the note says it explicitly — an "h_comm_size" key in sbd_config is now silently ignored.
• carryover_type defaults to 0 (Separate SQD parameters from SBD parameters, and expose the ones that were unreachable #28): a speedup, with bit-identical energies.
• SBD_BUILD_BACKEND=both is rejected in favor of all (Conda-friendly builds, AMD GPU support, and macOS OpenMP #23).
• Backends load one per process, with OMP_TARGET_OFFLOAD defaulted to MANDATORY (Conda-friendly builds, AMD GPU support, and macOS OpenMP #23) so a silent host fallback becomes an error.
• pybind11 is no longer a runtime dependency (Drop pybind11 from the runtime dependencies #33).

Scope

Examples get one combined note rather than one per pull request: they are scripts in the repository, not importable API. It records the parameter regrouping, the dropped Hartree-Fock seed and the new enlarge-subspace driver without itemizing seven pull requests. CI and documentation-only changes get no notes.

Three changes are deliberately omitted as invisible to a 1.6.1 user: the importlib.metadata version switch (#16), the vendored upstream bump (#24 — the makestring template instantiation preserves the exposed signature), and the internal GPU build-flag cleanups (#17, #25).

Verification

sphinx-build -W --keep-going, the same command tox -e docs runs, succeeds with zero warnings. All 19 notes render under the correct New Features / Upgrade Notes / Bug Fixes headings, and all 22 :func: cross-references resolve to real anchors — 22 roles, 22 links, so none failed silently. reno lint passes.

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This pull request was generated by Claude Opus 5 under my guidance.