Add deterministic practiceLog pruning for Bengali progress storage

[robirahman]

Motivation

• Prevent unbounded growth of progress.practiceLog by keeping the stored daily XP history bounded and deterministic.
• Ensure stored keys are strict YYYY-MM-DD dates and remove invalid or stale entries so progress storage is predictable and safe to persist.

Description

• Add prunePracticeLog(log, maxDays = 365) which enforces YYYY-MM-DD keys, validates calendar dates, computes a UTC-based cutoff window, sorts keys deterministically, and returns a pruned object containing only recent entries.
• Run pruning during load inside _loadProgressLS and mark the loaded data as mutated so migrated/pruned logs are persisted back with _saveProgressLS.
• Run pruning immediately before writing in _saveProgressLS so writes remain bounded over time.

Testing

• Ran node --check bengali/app.js which completed successfully indicating the updated file parses without syntax errors.

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Codex Task

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