Pretty-print circuits and patterns

[thierry-martinez]

• Add dedicated pretty-printing functions for patterns.
• Improve str() and repr() for circuits, patterns, and core data types (Sign, Axis, Plane, Command, Instruction).

This is a focused extract from PR #245, removing any external dependencies:

• Render patterns as ASCII, Unicode, or LaTeX snippets (PDF/image output removed).
• Provide evaluable representations for patterns, circuits, and other core types.

Per Shinichi’s suggestion (#245 (comment)), move most code from pattern.py into a new module pretty_print.py.

Per EarlMilktea’s suggestion (#245 (comment)), add type checking to pretty_print.py and its tests (test_pretty_print.py).

[codecov]

Codecov Report

Attention: Patch coverage is 94.40789% with 17 lines in your changes missing coverage. Please review.

Project coverage is 81.36%. Comparing base (f657712) to head (d111615).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
graphix/pretty_print.py	93.66%	9 Missing ⚠️
graphix/pattern.py	89.47%	4 Missing ⚠️
graphix/transpiler.py	93.02%	3 Missing ⚠️
graphix/instruction.py	98.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #277      +/-   ##
==========================================
+ Coverage   80.34%   81.36%   +1.02%     
==========================================
  Files          35       36       +1     
  Lines        5245     5432     +187     
==========================================
+ Hits         4214     4420     +206     
+ Misses       1031     1012      -19     

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