Make v1 the default arithmetic convention (Stage 2)

Start with arithmetics-design/convention.md and arithmetics-design/open-items.md, then inspect the options["semantics"] implementation and the migration guide at doc/migrating-to-v1.rst. Check the warning-channel decision in #738 and downstream readiness in #857 before changing the default. Done means the pre-1.0 release is selected, legacy remains an opt-out with its deprecation warning, and related docs and release notes are updated.

[!NOTE]
The following was generated by AI.

Tracks Stage 2 of the v1 arithmetic-convention rollout (opt-in → default → 1.0). v1 shipped opt-in in #717 (linopy.options["semantics"] = "v1", legacy the default). This issue is the decision and work to flip the default to v1.

Prerequisites (done)

• v1 opt-in released — #717
• Transition surface complete — every legacy→v1 divergence warns under legacy (LinopySemanticsWarning, naming the fix)
• Migration guide — doc/migrating-to-v1.rst

To make v1 the default

• Pick the release for the flip — a pre-1.0 minor, so 1.0 can ship legacy-free (#846). The flip must precede 1.0; otherwise it collides with the removal in a single release, leaving no window where v1 is the default and legacy is still opt-out.
• Flip the options["semantics"] default to "v1". Legacy stays reachable via options["semantics"] = "legacy" from the flip through to 1.0 (however many minors that spans), and is removed at 1.0.
• Add a dedicated opt-out deprecation warning: once v1 is the default, setting options["semantics"] = "legacy" emits a "legacy is deprecated, removed at 1.0" signal — distinct from the per-site LinopySemanticsWarning (which flags behaviour changes, not "on borrowed time"). Use whichever unified warning channel #738 settles on.
• Gate the flip on downstream readiness — the check-in in #857 (PyPSA, PyPSA-Eur, Calliope, flixopt) so they opt in / pin before it lands.
• Announce ahead of time: changelog note + downstream heads-up.
• Re-frame any docs/notebooks that still describe legacy as the default.

The LinopySemanticsWarning is a FutureWarning firing since the opt-in release (#717), so the deprecation runway is already long by the time of the flip.

Legacy removal is a separate 1.0 step (arithmetics-design/legacy-removal.md). Spec: arithmetics-design/convention.md; rollout: arithmetics-design/open-items.md.