Add Excel-compatible XIRR wrapper

[Thorium]

All the finance people (and chatbots) seem to be madly in love with XIRR functions. So here you go...

This PR adds Excel-compatible XIRR (Extended Internal Rate of Return) functionality to the FSharp.Finance.Personal library, enabling calculation of annualized returns for irregular cash flow schedules.

Instead of re-inventing the wheel, this adds a new dependency to fsprojects/ExcelFinancialFunctions.

Features Added

Core XIRR Module (src/Xirr.fs)

• Xirr.xirr: Calculate XIRR with Excel's default guess of 0.1 (10%)
• Xirr.xirrG: Calculate XIRR with custom initial guess parameter
• Xirr.tryXirr: Safe calculation returning Result<decimal, string> for error handling

Excel Compatibility

Uses the ExcelFinancialFunctions library (v3.2.0) to ensure complete parity with Excel's XIRR function, including:

• Identical convergence algorithm and precision
• Default guess value of 0.1 matching Excel behavior
• Same sign convention (negative=outflows, positive=inflows)

Input Validation

All functions validate that:

• At least two cashflows are provided
• At least one positive and one negative cashflow exists
• Dates are not all identical
• Throws ArgumentException for invalid inputs (except tryXirr which returns Result.Error)

Usage Example

open System
open FSharp.Finance.Personal

// Basic investment scenario
let cashflows = [ DateTime(2024, 1, 1), -10000m; DateTime(2025, 1, 1), 11000m ]
let rate = Xirr.xirr cashflows
printfn "XIRR = %.2f%%" (rate * 100m)  // Output: ~10.00%

// Salary advance scenario  
let salaryAdvance = [ DateTime(2024, 1, 1), 1000m; DateTime(2024, 1, 31), -1030m ]
let advanceRate = Xirr.xirr salaryAdvance  // ~43% annualized

// Safe error handling
match Xirr.tryXirr cashflows with
| Ok rate -> printfn "Success: %.2f%%" (rate * 100m)
| Error msg -> printfn "Failed: %s" msg

Testing

Comprehensive test suite (tests/XirrTests.fs) covers:

• Basic investment scenarios with ~10% returns
• Short-term salary advance calculations (~43% annualized)
• Input validation for mixed signs requirement
• Consistency between xirr and xirrG with same guess
• Edge cases and error conditions

All 7 test cases pass successfully when run in isolation.

Documentation

• Example script: docs/exampleXirr.fsx with comprehensive usage examples
• README update: Added XIRR section with quick start example
• XML documentation: Detailed function documentation with sign convention explanations

Integration Notes

• Non-breaking: All changes are additive, no existing functionality modified
• Version: Left at 2.4.7 to coordinate with other pending PRs
• Dependencies: Adds ExcelFinancialFunctions package dependency
• Isolation: Module is self-contained with no coupling to existing domain modules

The implementation follows the repository's existing patterns and conventions, using [<RequireQualifiedAccess>] module attributes and consistent error handling approaches.