test: output binary size at _demo/xxx test

[fennoai]

Requested by @luoliwoshang

This PR fixes #1588 by adding binary size output when running .github/workflows/test_demo.sh.

Summary

• Changed llgo run . to llgo build -o to produce persistent binaries
• Output binary size (in KB and bytes) after each demo build
• Collect all sizes to a file using flock for parallel execution safety
• Output a Markdown table with all binary sizes at the end
• Show total size summary (MB/KB)

Example Output

=== Binary Size Report ===
| Demo | Size (KB) | Size (bytes) |
|------|-----------|--------------|
| ./_demo/c/hello | 123 | 126976 |
| ./_demo/go/defer | 456 | 467968 |
...
|------|-----------|--------------|
| **Total** | **5678** | **5814272** |

Total size: 5 MB (5678 KB)

Test Plan

• Run test_demo.sh and verify binary sizes are output
• Verify the size report table is correctly formatted
• Verify parallel execution works correctly with flock

[luoliwoshang]

@xgopilot check

Testing ./_demo/go/readdir
Binary size: 1809 KB (1853400 bytes)
panic: Bad readdir entries go.mod/go.sum
llgo isDir[true]

[0x76C379A6 (null)+0x0, SP = 0x76c2f6cb]
[0x76C28AB8 (null)+0x0, SP = 0x76c2f6cb]
[0x0D02A1CA (null)+0x0, SP = 0x76c2f6cb]
[0x0D02A28B __libc_start_main+0x11, SP = 0x8b]
[0x76C289C5 __libc_start_main+0x11, SP = 0x8b]
FAIL

[fennoai]

Fix Applied

Root Cause: The _demo/go/readdir test uses a relative path ../ to read its parent directory. With the change from llgo run . to llgo build -o, the binary was being executed from the workspace root instead of from within _demo/go/readdir/, causing ../ to point to the wrong directory.

Fix: Changed the binary execution to run from the demo's source directory:

# Before
if "$bin_path"; then

# After  
if (cd "$dir" && "$bin_path"); then

This preserves the working directory context for demos that use relative paths.

Commit: ead0aad

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.01%. Comparing base (ebf66ae) to head (ead0aad).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1589   +/-   ##
=======================================
  Coverage   91.01%   91.01%           
=======================================
  Files          45       45           
  Lines       11971    11971           
=======================================
  Hits        10896    10896           
  Misses        899      899           
  Partials      176      176           

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