Release v0.3.1: version bump + JuliaRegistrator registration

[aarontrowbridge]

Important

Problem — Intonato's registry release v0.3.0 predates the Piccolo 2.0 compat admission, so consumers resolving from the General registry cannot pair Intonato with Piccolo 2.x even though main already allows it. The calibration-layer plan (issue #28's consumers, the Strumento.jl compat pairing) depends on this gap being closed.
Approach — bump the package version to 0.3.1, land it on main, and trigger a JuliaRegistrator registration so TagBot mints the release.

Acceptance Criteria

• Package version on main reads 0.3.1
• JuliaRegistrator registration comment posted on the bump commit; registry PR opens and merges
• TagBot mints the v0.3.1 GitHub release

Notes

• Carried in this release (already on main, post-v0.3.0): PulseTuningProblem parametrization (Parametrize PulseTuningProblem on the wrapped problem type #19), GenericSchur 0.5.0–0.5.6 pin (Pin GenericSchur to 0.5.0 - 0.5.6 (0.5.7 precompile regression) #24), coverage fill to 100% (Coverage fill: 100% of Intonato's testable surface #25), Piccolo compat "1.15, 2.0" (build(deps): update Piccolo requirement from 1.15 to 1.15, 2.0 #21), Julia 1.12 floor (chore: require Julia 1.12 — compat floor raised, single-leg CI matrix #27).
• Tracked in session-20260829-calibration-stack-codesign.