Update README to reflect current module structure and O(1) correctness focus

[Copilot]

README was stale: referenced removed modules (tree model, iterator/state-machine), listed only 2 of 6 Lean files, had a wrong path to the top-level module, and described the repo's goal as "formalization and study" rather than what it actually is.

Changes

• Description: Reframes the repo goal from "formalization and study" to proving correctness of the new O(1) segment-based implementation
• Overview: Replaces tree/iterator bullet points with accurate segment-based implementation bullets (SegmentSequence, SegmentedState, trailing zeros lemmas)
• Modules section: Expands from 2 to all 6 current files — Size, TrailingZeros, Basic, SegmentSequence, SegmentedState, Equivalence — each with accurate descriptions
• Root module imports: Corrects the listed imports to match LubySequence.lean (Basic, SegmentSequence, SegmentedState, Equivalence)
• Top-level module path: Fixes LubySequence/LubySequence.lean → LubySequence.lean

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