chore: release v0.2.1

[MasanoriKanamaru]

Summary

• Bump version to 0.2.1
• Update CHANGELOG.md with v0.2.1 entries
• Update ROADMAP.md: mark v0.2.1 tasks complete, add v0.3.0 ThermoParams redesign plan
• Update README.md: rewrite Basic Usage section to reflect v0.2.x API; add v0.2.1 patch notes to What's New

Changes in v0.2.1

• *Ephemerides constructors now accept AbstractRange for times (PR feat: accept AbstractRange as times in *Ephemerides constructors #215)
• *Ephemerides constructors auto-convert plain arrays to SVector/SMatrix (PR feat: auto-convert plain Arrays to StaticArrays in *Ephemerides constructors #214)
• Internal: renamed energy_in/energy_out to absorbed_power/emitted_power (PR refactor: rename energy_in/energy_out to integrate_absorbed/emitted_power #212)
• Internal: test prefix cleanup (PR refactor: remove AsteroidThermoPhysicalModels. prefix from exported symbols in tests #213)

No migration required from v0.2.0.

Test plan

• CI passes on this branch
• Version in Project.toml is 0.2.1
• ## [0.2.1] section present in CHANGELOG.md

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.59%. Comparing base (903edfb) to head (605b5d2).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #216   +/-   ##
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  Coverage   96.59%   96.59%           
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  Files          14       14           
  Lines         704      704           
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  Hits          680      680           
  Misses         24       24           

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