Fix (*os.File).Stat() returning Size() = 0

[xgopilot]

Requested by @cpunion

Fixes #1503
Fixes #1475

Summary

Fixed a bug where (*os.File).Stat() returns Size() = 0 on an open file, while standard Go returns the correct file size.

This fix also resolves issue #1475, enabling llama2-go to compile and run correctly with llgo. The issue was that llama2-go uses encoding/binary to read binary model files, which internally relies on (*os.File).Stat() to get file sizes.

Root Cause

In runtime/internal/lib/os/stat_unix.go:17-27, the File.Stat() method calls os.Fstat() to get the syscall stat structure but doesn't call fillFileStatFromSys() to copy the fields (including size) from the syscall structure to the fileStat structure. This caused all fields including size to remain at their zero values.

In contrast, statNolog() and lstatNolog() properly call fillFileStatFromSys() after getting the syscall data.

Changes

• Added fillFileStatFromSys(&fs, f.name) call in File.Stat() after os.Fstat()
• Created test case in _cmptest/filestat/ to verify the fix

Test Plan

• Created and verified test case with standard Go
• Ran project test suite (go test ./cl/...)
• Formatted code with go fmt ./...
• Verified fix resolves llama2-go compatibility issue

Expected Behavior

Both os.Stat() and file.Stat() now correctly return the file size:

os.Stat():   size = 400
file.Stat(): size = 400

Generated with codeagent

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.93%. Comparing base (05299c6) to head (21e4bfe).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1506   +/-   ##
=======================================
  Coverage   90.93%   90.93%           
=======================================
  Files          44       44           
  Lines       11525    11525           
=======================================
  Hits        10480    10480           
  Misses        883      883           
  Partials      162      162           

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