arm64: support register-extend operands

[cpunion]

Summary

• add parser support for arm64 register-extend operands like R2.UXTB
• lower register-extend operands in arm64 eval so CMP R2.UXTB, R5 translates directly
• add parser/eval/translate coverage for the new operand form

Why

Go 1.26's internal/bytealg/count_arm64.s uses CMP R2.UXTB, R5.
llgo was carrying a downstream source rewrite to normalize that operand before translation. This change moves the support into plan9asm so downstream users can translate the stdlib source as-is.

Testing

• go test ./...
• go test ./... -run 'TestStdlibInternalBytealg_ARM64_Compile'

[codecov]

Codecov Report

❌ Patch coverage is 92.15686% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.55%. Comparing base (33e8ba1) to head (e71ba40).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
arm64_eval.go	93.54%	1 Missing and 1 partial ⚠️
types.go	90.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #13      +/-   ##
==========================================
+ Coverage   69.48%   69.55%   +0.07%     
==========================================
  Files          57       57              
  Lines       16380    16431      +51     
==========================================
+ Hits        11382    11429      +47     
- Misses       3683     3685       +2     
- Partials     1315     1317       +2     

Flag	Coverage Δ
unittests	69.55% <92.15%> (+0.07%)	⬆️

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[gemini-code-assist]

Code Review

This pull request implements support for ARM64 register extensions (e.g., R2.UXTB) by introducing the OpRegExtend operand kind. The changes include updates to the operand parser, string representation, and the evaluation logic to generate appropriate truncation and extension IR. Feedback was provided to optimize the parseRegExtend function by avoiding redundant string transformations.

[fennoai]

Clean, well-scoped addition. The parser, eval, and String() all follow existing codebase patterns closely, and test coverage is solid (edge cases, round-trip, stdlib integration). Three minor items inline: a duplicated string expression in parseRegExtend, a missing // OpRegExtend annotation on Operand.Ext, and a missing doc comment on the new public ExtendOp type.