feat: add comprehensive energy unit conversion module

[AliAlimohammadi]

Description

This PR adds a new energy conversion module to the conversions library, enabling conversion between 70+ energy units across different measurement systems and scientific disciplines.

Implementation

Design Decisions

• SI-based conversions: All conversions route through joules (the SI base unit) as an intermediary, ensuring consistency and accuracy
• Two-step conversion: Follows the established pattern of to_joule() → joule_to_unit() for clarity and maintainability
• Type-safe enum API: Uses EnergyUnit enum matching the repository's temperature.rs pattern for compile-time safety
• Pattern matching: Consolidates equivalent units (e.g., Joule | WattSecond | NewtonMeter) for cleaner code

Supported Units (70+ units)

SI Units (8)

• Joule, kilojoule, megajoule, gigajoule
• Millijoule, microjoule, nanojoule, attojoule

Power-Time Units (6)

• Watt-second, watt-hour, kilowatt-second, kilowatt-hour
• Megawatt-hour, gigawatt-hour

Mechanical Units (3)

• Newton-meter, erg, dyne-centimeter

Calorie Variants (6)

• Nutritional calorie, nutritional kilocalorie
• IT calorie, IT kilocalorie
• Thermochemical calorie, thermochemical kilocalorie

Electron Volt Units (3)

• Electronvolt (eV), kiloelectronvolt (keV), megaelectronvolt (MeV)

British Thermal Units (3)

• BTU (IT), BTU (thermochemical), mega BTU (IT)

Imperial Force-Distance Units (7)

• Foot-pound, pound-force foot, pound-force inch
• Inch-pound, inch-ounce, ounce-force inch, poundal foot

Horsepower Units (2)

• Horsepower hour, metric horsepower hour

Metric Force Units (5)

• Kilogram-force meter, kilogram-force centimeter, kilopond meter
• Gram-force meter, gram-force centimeter

Therm Units (3)

• Therm, therm (EC), therm (US)

Specialized Units (7)

• Ton-hour refrigeration
• Fuel oil equivalent (kiloliter), fuel oil equivalent (barrel)
• Ton of TNT, kiloton, megaton, gigaton

Atomic Units (2)

• Hartree energy, Rydberg constant

Testing

Test Coverage

• ✅ 40+ comprehensive tests covering all major unit categories
• ✅ Round-trip conversions verify accuracy in both directions for 8 common units
• ✅ Same-unit conversions ensure identity operations work correctly
• ✅ Edge cases including zero values, negative values, and large values
• ✅ Cross-system conversions (e.g., SI ↔ Imperial, calories ↔ joules, eV ↔ joules)
• ✅ Unit equivalence tests verify that equivalent units (e.g., foot-pound vs pound-force foot) convert identically
• ✅ Real-world scenarios (food energy, electricity bills, explosive yields, atomic physics)

Test Organization

All tests use the approx_eq() helper for floating-point comparisons with appropriate tolerances, ensuring robust validation across the full range of energy scales from attojoules to gigatons.

Verification

cargo test energy::        # Run all energy tests
cargo clippy              # Zero warnings
cargo fmt --check         # Properly formatted

Code Quality

• Documentation: Comprehensive module and function documentation with usage examples
• Type Safety: Enum prevents invalid unit specifications at compile time
• Precision: Uses f64 for all calculations with appropriate tolerance checks (EPSILON = 1e-10)
• Consistency: Exactly matches existing temperature.rs module pattern
• No Dependencies: Uses only standard library
• Clean Code: Zero clippy warnings, properly formatted

Examples

use the_algorithms_rust::conversions::energy::{convert_energy, EnergyUnit};

// Electricity billing
let kwh_to_joules = convert_energy(100.0, EnergyUnit::KilowattHour, EnergyUnit::Joule);
// 100 kWh = 360,000,000 J

// Food nutrition
let calories = convert_energy(2000.0, EnergyUnit::CalorieNutritional, EnergyUnit::Kilojoule);
// 2000 Cal ≈ 8,374 kJ

// Physics calculations
let electron_volts = convert_energy(1.0, EnergyUnit::Joule, EnergyUnit::Electronvolt);
// 1 J ≈ 6.242×10¹⁸ eV

// Engineering applications
let btu_to_kwh = convert_energy(3412.0, EnergyUnit::BtuIT, EnergyUnit::KilowattHour);
// 3412 BTU ≈ 1 kWh

// Explosive energy
let megaton_to_gj = convert_energy(1.0, EnergyUnit::Megaton, EnergyUnit::Gigajoule);
// 1 megaton TNT = 4,184,000 GJ

References

All conversion factors are based on authoritative sources:

• Units of energy - Wikipedia
• Joule - Wikipedia
• Kilowatt-hour - Wikipedia
• Calorie - Wikipedia
• Electronvolt - Wikipedia
• British thermal unit - Wikipedia
• Foot-pound - Wikipedia
• Unit Converters

Checklist

• Code follows the repository's style guidelines
• Self-review of code completed
• Code is well-commented with comprehensive documentation
• Corresponding documentation added
• No new warnings generated (cargo clippy passes with zero warnings)
• All tests pass (cargo test shows 100% pass rate)
• Follows the same pattern as existing conversion modules (matches temperature.rs)
• Module added to mod.rs with proper exports

[AliAlimohammadi]

@siriak, this is ready to be merged.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 81.75439% with 52 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.98%. Comparing base (d0536ba) to head (4263563).

Files with missing lines	Patch %	Lines
src/conversions/energy.rs	81.75%	52 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1014      +/-   ##
==========================================
- Coverage   96.12%   95.98%   -0.14%     
==========================================
  Files         380      381       +1     
  Lines       28250    28535     +285     
==========================================
+ Hits        27154    27388     +234     
- Misses       1096     1147      +51     

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