Add finite-difference result type

[ohno]

Summary

• add ResultFiniteDifferenceMethod for both FDM solve paths
• preserve property access for energies, matrices, coefficients, and wavefunctions
• move the existing solver report into the result's string and show methods
• document and test full, compact, and trial-wavefunction results

Impact

FDM calculations now return a named result type without changing the existing result.E-style interface.

Validation

• Pkg.test() — 559/559 tests passed
• complete Documenter build passed
• no method ambiguities detected
• git diff --check

Closes #3

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This pull request was developed with assistance from Codex using GPT-5.6 Sol with High reasoning effort.