docs(arithmetics): track open-items.md; mark #744 resolved (#744)

Commit the v1-rollout checklist (previously untracked) as the shared status
tracker, refreshed to the current state: the one open design decision (#744,
MultiIndex storage) is resolved — v1 disallows MI — and the remaining work is
organised by goals.md's three stages (opt-in / default / 1.0). The
transition-surface-complete + changelog items are placed at stage 1, matching
goals.md step 1 (warn on legacy, raise on v1).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: FBumann

arithmetics-design/open-items.md

@@ -6,26 +6,29 @@ coordinate-alignment intro rules all have a v1 implementation. The high-level
 schedule lives in [`goals.md`](goals.md) (opt-in → default → 1.0); this file
 tracks the concrete items per stage.
 
-One open **design** decision remains — [#744] MultiIndex storage. No open
-arithmetic-rule questions.
+The one open **design** decision — [#744] MultiIndex storage — is **resolved**:
+v1 disallows first-class `pd.MultiIndex` (a flat dim + auxiliary level coords),
+implemented in [#803]. No open arithmetic-rule questions remain; everything left
+is rollout + cleanup.
 
 ## Stage 1 — release v1 (opt-in)
 
 Legacy stays the default. The transition surface must already be **complete**
 here ([`goals.md`] step 1: *warn on legacy, raise on v1*).
 
-- [ ] **Resolve [#744] — MultiIndex storage — before #717 merges.** First-class
-  `pd.MultiIndex` vs. a flat dim + auxiliary level coords. §11's
-  stacked-MultiIndex rule and the storage of a `snapshot`-style
-  `(period, timestep)` dim both depend on it — and §11 **ships in #717**, so
-  changing the model after v1 is released would change observable §11 behaviour.
-  Must be locked (and its implementation in the same release cut) before #717
-  ships v1, not deferred to the default flip.
+- [x] **Resolve [#744] — MultiIndex storage — before #717 merges.** Resolved:
+  v1 disallows first-class `pd.MultiIndex` (flat dim + auxiliary level coords).
+  §11's stacked-MultiIndex rule and the storage of a `snapshot`-style
+  `(period, timestep)` dim both depend on this, and §11 **ships in #717** — so
+  the implementation ([#803]) must land in the **same release cut** as #717,
+  else released §11 behaviour would change when it lands.
 - [ ] Land [#717] (v1 semantics) → `master`
+- [ ] Land [#803] (v1 MultiIndex drop) with it — same cut, so §11 ships final
 - [ ] **Transition surface complete** — every behaviour-change site raises under
   v1 *and* warns under legacy (`warn_legacy`, naming the fix). This is the
   "no silent change" guarantee ([`goals.md`] transitioning goal #3): shipping v1
-  with a gap would silently change any model that opts in.
+  with a gap would silently change any model that opts in. Includes #803's
+  MultiIndex rejects and the multi-key-`groupby`-flat change as new fork sites.
 - [ ] Changelog note — v1 available via `options['semantics'] = 'v1'`; legacy
   remains the default; link [`convention.md`].
 
@@ -40,8 +43,9 @@ here ([`goals.md`] step 1: *warn on legacy, raise on v1*).
 - [ ] The **strip**: the concentrated MI machinery (the `alignment.py`
   level-projection subsystem, netcdf MI (de)serialize) *and* the scattered
   surface (`assign_multiindex_safe` ×39, `isinstance(MultiIndex)` guards, MI
-  branches) — plus every other `grep "LEGACY: remove at 1.0"` marker.
-  Dependency-ordered checklist in [`legacy-removal.md`].
+  branches) — all kept live for legacy until now — plus every other
+  `grep "LEGACY: remove at 1.0"` marker. Dependency-ordered checklist in
+  [`legacy-removal.md`].
 - [ ] Reframe [`convention.md`] / [`goals.md`] — drop the "v1"/legacy framing
   once there is only one convention.
 
@@ -55,15 +59,19 @@ here ([`goals.md`] step 1: *warn on legacy, raise on v1*).
 
 ## Decisions
 
-- [ ] **[#744] — MultiIndex storage** — the one open design decision; gates the
-  **v1 release** (§11 ships in #717 and depends on it), not just the default flip.
+- [x] **[#744] — MultiIndex storage** → v1 disallows MI (flat + aux coords),
+  implemented in [#803]; must land in the same release cut as #717 (§11 depends
+  on it).
+- [x] **Legacy warnings live from the opt-in release** → yes; the transition
+  surface is complete at stage 1, not deferred.
 - [ ] **When v1 becomes the default** — pick the release; gated on the migration
   guide.
 
 <!-- references -->
 [#744]: https://github.com/PyPSA/linopy/issues/744
 [#714]: https://github.com/PyPSA/linopy/issues/714
 [#717]: https://github.com/PyPSA/linopy/pull/717
+[#803]: https://github.com/PyPSA/linopy/pull/803
 [`goals.md`]: goals.md
 [`docs-plan.md`]: docs-plan.md
 [`legacy-removal.md`]: legacy-removal.md