Add opt-in correction for zero-duration survey activities

[FlxPo]

Motivation

Rounded survey clocks can produce zero-duration activities when one trip arrives at the same reported time the next trip departs. These zeros distort downstream activity-duration distributions.

Changes

• Add the opt-in correct_zero_durations survey parameter.
• Fit corrections reproducibly from the survey using weighted, interval-censored Gamma distributions.
• Apply corrections during EMP survey creation and include the flag in the cache hash.
• Share available travel time across adjacent corrections so trips cannot become negative.
• Raise ValueError for ENTD until its parser loads and standardizes the trip times already present in the survey.
• Add tests and user/method documentation.

Example (if relevant)

survey = mobility.EMPMobilitySurvey(
    correct_zero_durations=True,
)

The feature is disabled by default and does not require an external CSV.

AI-assisted contribution

• AI used for substantial parts of this PR

If substantial, briefly describe:

• Scope of AI-assisted content: implementation, tests, refactoring, and documentation.
• What you reviewed or changed: survey API, correction method, capacity constraints, and ENTD handling.
• How you validated it: focused tests, downstream MRMT tests, strict documentation build, and compilation checks.

Checklist

• I have reviewed all code in this PR
• I understand the code and can maintain it
• I added or ran appropriate tests/checks for the changed behavior

[codecov]

Codecov Report

❌ Patch coverage is 95.10490% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.97%. Comparing base (8795552) to head (cb52d4d).

Files with missing lines	Patch %	Lines
mobility/surveys/zero_duration_activities.py	94.53%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #402      +/-   ##
==========================================
+ Coverage   82.86%   82.97%   +0.10%     
==========================================
  Files         231      232       +1     
  Lines       15835    15978     +143     
==========================================
+ Hits        13121    13257     +136     
- Misses       2714     2721       +7     

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