Update Tuist and migrate the Example package dependency

[Kyle-Ye]

Summary

• update the root and Example Tuist pins to 4.203.0-rc.1
• declare OpenAttributeGraph as a native local Swift package in the Example project
• remove the obsolete Tuist external-dependency entries and lockfile

Motivation

The existing Example package declaration no longer generates after updating to Tuist 4.203.0-rc.1. Declaring OpenAttributeGraph as a native local Swift package restores project generation.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 32.10%. Comparing base (7877791) to head (b307fa2).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #232      +/-   ##
==========================================
- Coverage   32.17%   32.10%   -0.08%     
==========================================
  Files          71       71              
  Lines        2601     2601              
==========================================
- Hits          837      835       -2     
- Misses       1764     1766       +2     

Flag	Coverage Δ
ios	32.11% <ø> (-0.08%)	⬇️
macos	10.15% <ø> (ø)
ubuntu	12.03% <ø> (ø)

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see 1 file with indirect coverage changes

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