build: extend source patch branch to go1.26 CI coverage

[cpunion]

Summary

• extend LLGo workflow coverage to go1.26.0
• keep source patch usage narrow and add internal/sync as the main go1.26 source patch
• add the minimal go1.26 runtime/reflect/crypto compat needed to boot hello and initial stdlib tests

Notes

• this branch is based on feat/source-patch-proto and therefore currently includes the source-patch prototype changes from build: prototype source patches for stdlib compat #1758
• go1.26 fixes here were added incrementally from local smoke tests instead of bulk-copying the old [Don't Merge] runtime: split darwin syscall shims for go1.26 and expand CI coverage #1753 patch set

[gemini-code-assist]

Code Review

This pull request implements a source patching system to facilitate compatibility with Go 1.26, allowing the build process to skip or override standard library declarations using AST-based filtering and specialized directives. It introduces several compatibility shims for runtime features like sync.HashTrieMap, iter, and reflect, while updating the build logic to handle these overlays. Feedback was provided regarding the fragility of using copy with bytes.Replace during directive sanitization, suggesting a more robust byte-level manipulation to avoid issues if replacement lengths differ.

[codecov]

Codecov Report

❌ Patch coverage is 55.26316% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.16%. Comparing base (2075aa9) to head (6b1e842).
⚠️ Report is 22 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/plan9asm/translate.go	0.00%	9 Missing and 1 partial ⚠️
internal/env/env.go	62.50%	5 Missing and 1 partial ⚠️
cl/instr.go	91.66%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #1759    +/-   ##
========================================
  Coverage   93.16%   93.16%            
========================================
  Files          48       48            
  Lines       13349    13616   +267     
========================================
+ Hits        12437    12686   +249     
- Misses        726      742    +16     
- Partials      186      188     +2     

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