[Justin Chu (justinchuby)]

Summary

• Treat graph-input initializer tensors as overridable defaults, not runtime constants, without clearing defaults or changing input signatures.
• Guard literal matching and constant-based shape, Slice, padding, Min/Max, bias, ScatterND, function-extraction, and opt-in ORT fusion paths using the existing constant helper.
• Add ORT regressions with omitted defaults and explicit overrides, true-constant positive controls, nested graph captures, and per-parameter guards. Correct Min/Max tests that expected defaults to be folded.

Separate from constant-folder changes in #3189 and unresolved-attribute handling in #3191. The matcher change does not touch #3168's candidate enumeration.

Validation

• Rewriter and optimizer suites: 715 passed, 3 skipped; 102 subtests passed.
• Ruff lint/format and diff checks pass; helper and new tests pass mypy.
• Public optimizer Add/Sub/Mul/Div preserve runtime overrides and original ModelProto defaults; true initializer and Constant operands still simplify.

[codecov]

Codecov Report

❌ Patch coverage is 96.31579% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.96%. Comparing base (f51083c) to head (b52a6ae).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...nxscript/rewriter/_overridable_initializer_test.py	98.52%	1 Missing and 1 partial ⚠️
onnxscript/rewriter/ort_fusions/cos_sin_cache.py	50.00%	1 Missing and 1 partial ⚠️
...ipt/rewriter/ort_fusions/fused_matmul_rule_sets.py	33.33%	0 Missing and 2 partials ⚠️
onnxscript/rewriter/rules/common/_basic_rules.py	75.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3195      +/-   ##
==========================================
+ Coverage   73.77%   73.96%   +0.18%     
==========================================
  Files         270      271       +1     
  Lines       33876    34023     +147     
  Branches     3243     3262      +19     
==========================================
+ Hits        24993    25166     +173     
+ Misses       7829     7804      -25     
+ Partials     1054     1053       -1     

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