docs(classical-mechanics): add TODO to split harmonic oscillator solution

[nateabr]

Adds a module-level TODO to HarmonicOscillator/Solution.lean recording the intended follow-up split discussed after #1308.

The TODO keeps Solution.lean as the umbrella import and records the proposed organization:

• Solution.Basic for trajectory construction and equation-of-motion facts;
• Solution.Energy for energy-related lemmas;
• Solution.InitialData for alternative initial-condition parametrizations;
• Solution.AmplitudePhase for the amplitude-phase normal form;
• Solution.SpecialTimes for velocity-zero times, turning points, and zero crossings;
• Solution.Periodicity for period and recurrence facts.

This is documentation only; no declarations or proofs are changed.

[github-actions]

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[jstoobysmith]

Would remove this.

[nateabr]

sounds good, ive made the change.

[nateabr]

-awaiting-author

[jstoobysmith]

Approved - will merge when passes CI.