Add heterodyne measurement interface

[arnavk23]

Implement heterodyne measurement for Gaussian states:

• Heterodyne struct for storing measurement results and output state
• Support for both QuadPairBasis and QuadBlockBasis
• Simultaneous measurement of both quadratures (unit-gain)
• rand() support for sampling heterodyne measurement outcomes

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[codecov]

Codecov Report

❌ Patch coverage is 96.37306% with 7 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@8a15976). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
src/heterodyne.jl	96.37%	7 Missing ⚠️

Additional details and impacted files

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  Coverage        ?   97.12%           
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  Files           ?       20           
  Lines           ?     2816           
  Branches        ?        0           
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  Hits            ?     2735           
  Misses          ?       81           
  Partials        ?        0           

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