docs: translate README and godoc comments to English for awesome-go submission

[zx06]

Summary

Prepare for awesome-go submission by translating all documentation to English as required by the contribution guidelines.

Changes

• README.md: Translated to English (primary language)
• README_zh.md: Preserved Chinese version with cross-link to English README
• pkg.go.dev badge: Added to README (required by awesome-go)
• Godoc comments: Translated all Chinese comments to English across 22 .go files
• Package-level docs: Added missing package doc comments for all internal packages
• cmd/xsql/doc.go: New file with package-level documentation

Verification

• go build ./... ✅
• go vet ./... ✅
• All unit tests pass ✅
• 0 Chinese comments remaining in non-test .go files ✅

Context

awesome-go requires:

• Documentation in English (README + pkg.go.dev doc comments)
• pkg.go.dev, Go Report Card, and Codecov badges
• Repository age ≥ 5 months (first commit: 2026-01-29, eligible ~July 2026)

This PR addresses all documentation requirements. The actual awesome-go PR will be submitted once the repo meets the 5-month maturity threshold.

[sonarqubecloud]

Quality Gate failed

Failed conditions
3.4% Duplication on New Code (required ≤ 3%)

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 80.92%. Comparing base (4048289) to head (73ea3bf).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
internal/output/writer.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #36   +/-   ##
=======================================
  Coverage   80.92%   80.92%           
=======================================
  Files          40       40           
  Lines        2726     2726           
=======================================
  Hits         2206     2206           
  Misses        397      397           
  Partials      123      123           

Flag	Coverage Δ
e2e	44.09% <25.00%> (ø)
integration	44.09% <25.00%> (ø)
unittests	68.23% <75.00%> (ø)

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