Add inPlace flag to time series operations for mutable transformations

[Copilot]

Implements mutable time series operations by adding an optional inPlace parameter (similar to pandas DataFrame) to avoid performance issues with large time series transformations.

Changes Made

Core Infrastructure

• Modified IndexedSeries base class to support controlled mutability during inPlace operations
• Added EnableMutation()/DisableMutation() and UpdateInPlace() protected methods
• Properly handles Start/End/IsEmpty property updates during mutations

New inPlace Method Overloads

All methods maintain backwards compatibility (inPlace defaults to false):

// Transform operations
ts.Apply(x => x * 2, inPlace: true);              // Modify in place
ts.ApplyValues(x => x + 5, inPlace: true);        // Transform non-null values

// Join operations  
ts.JoinLeft(other, (l, r) => l + r, inPlace: true);     // 3 overloads supported
ts.JoinLeft(other, JoinOperation.Add, inPlace: true);

// Slice operations
ts.Slice(startIndex: 2, count: 5, inPlace: true);

Convenience Methods

Added fluent API methods for common operations with method chaining:

// Scalar operations
ts.AddInPlace(10).MultiplyInPlace(2).SubtractInPlace(5);

// Time series operations  
ts.AddInPlace(otherSeries).DivideInPlace(anotherSeries);

Comprehensive Testing

• Created TimeSeriesInPlaceTest with 13 test methods covering all scenarios
• Tests verify reference equality, immutable behavior preservation, null handling, and edge cases

Usage Examples

Before (creates new instances):

var ts = TimeSeries.Factory.FromValue(100, startDate, 1000000, Period.Day);
ts = ts.Apply(x => x * 2);        // Creates new 1M element series
ts = ts.Apply(x => x + 10);       // Creates another new 1M element series  
ts = ts.JoinLeft(other, op);      // Creates yet another new series

After (modifies in place):

var ts = TimeSeries.Factory.FromValue(100, startDate, 1000000, Period.Day);
ts.Apply(x => x * 2, inPlace: true)      // Modifies existing series
  .Apply(x => x + 10, inPlace: true)     // No new allocations
  .JoinLeft(other, op, inPlace: true);   // Still the same instance

Benefits

• Performance: Eliminates object allocations for large time series operations
• Memory Efficiency: Reduces memory pressure and GC overhead
• Backwards Compatibility: All existing code continues to work unchanged
• Pandas-like API: Familiar interface for users coming from pandas DataFrames
• Type Safety: Maintains all existing validation and type safety

Fixes #1.

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