chore: bump version to 1.0.0 and declare stable API

[Copilot]

0.1.1 signals an unstable, pre-production library. This PR promotes jQuantStats to 1.0.0 and formally documents the API surface covered by semver guarantees.

Changes

• pyproject.toml — version 0.1.1 → 1.0.0
• uv.lock — regenerated for new version
• CHANGELOG.md — new [1.0.0] entry with a Stable API section that enumerates every semver-protected surface:
  • Top-level exports: Portfolio, build_data, __version__
  • Portfolio factory methods (.from_cash_position, .from_risk_position) and properties (.stats, .plots, .report, .data)
  • build_data signature
  • Full Stats API (30+ methods)
  • Plots API (plot_snapshot)
  • Report API (metrics)

Original prompt

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This section details on the original issue you should resolve

<issue_title>Plan 1.0.0 milestone — Current 0.1.1 signals instability; define and document what constitutes stable API for a 1.0 bump</issue_title>
<issue_description></issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Plan 1.0.0 milestone — Current 0.1.1 signals instability; define and document what constitutes stable API for a 1.0 bump #405

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