ci: run stdlib corpus gate across Go versions

[cpunion]

Summary

Run the stdlib asm corpus gate across the same Go version range already covered by the go test matrix, and run that gate on both CI host platforms we care about: ubuntu-latest and macos-latest.

Why

The existing workflow only ran scripts/check-stdlib-corpus.sh under Go 1.26.x on Ubuntu.
That left an older-stdlib gap: code paths exercised by LLGo demos under Go 1.21.13 could still pull in asm syntax that the current corpus gate never checked.

This change makes that gap explicit by adding a dedicated stdlib-corpus matrix job.

What changed

• Keep build as the single-version Go 1.26.x build job.
• Add a separate stdlib-corpus matrix job for:
  • host OS:
    • ubuntu-latest
    • macos-latest
  • Go versions:
    • 1.21.x
    • 1.22.x
    • 1.23.x
    • 1.24.x
    • 1.25.x
    • 1.26.x
• The corpus script itself still cross-compiles the stdlib asm corpus for:
  • linux/amd64
  • linux/arm64
  • darwin/amd64
  • darwin/arm64

Local verification

• GOTOOLCHAIN=go1.26.1 ./scripts/check-stdlib-corpus.sh
  • passes on macOS host
• GOTOOLCHAIN=go1.21.13 ./scripts/check-stdlib-corpus.sh
  • currently fails on linux/amd64
  • this is expected and is the point of the change: expose the missing old-stdlib coverage in CI

[gemini-code-assist]

Note

Gemini is unable to generate a summary for this pull request due to the file types involved not being currently supported.

[fennoai]

Good approach to close the older-stdlib coverage gap. Two things to address: (1) the '1.26.x' matrix entry duplicates the corpus gate already in the build job, and (2) the PR description explicitly states Go 1.21.x currently fails — if stdlib-corpus is configured as a required status check, this will block all subsequent PRs until the gap is fixed.

[codecov]

Codecov Report

❌ Patch coverage is 55.55556% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 32.44%. Comparing base (f1146f5) to head (15b2d9c).
⚠️ Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
preprocess.go	55.55%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main       #9   +/-   ##
=======================================
  Coverage   32.44%   32.44%           
=======================================
  Files          41       41           
  Lines       12640    12646    +6     
=======================================
+ Hits         4101     4103    +2     
- Misses       7806     7809    +3     
- Partials      733      734    +1     

Flag	Coverage Δ
unittests	32.44% <55.55%> (+<0.01%)	⬆️

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