fix(build): truncate search index descriptions by rune count not bytes

[greynewell]

Summary

• generateSearchIndex was using len(desc) > 120 / desc[:120] — byte operations — to cap description length before writing the search index JSON
• For multi-byte UTF-8 characters (é, ñ, ü, CJK, emoji), slicing at byte position 120 can land inside a character's byte sequence, producing an invalid byte string
• json.Marshal silently replaces invalid UTF-8 with U+FFFD (replacement character \uFFFD) — so the search index would contain corrupted descriptions with no error returned
• Fix: convert to []rune, check/slice by rune count, convert back to string

Regression test added in build_test.go:

• Short description written verbatim
• Long ASCII description truncated to exactly 120 runes
• 121-rune string of é (2 bytes each) truncated at rune boundary → valid UTF-8
• Search disabled → no file written

Test plan

• go test ./internal/archdocs/pssg/build/... passes
• go build ./... passes
• TestGenerateSearchIndex_MultiByteDescriptionTruncation fails on the old byte-slice code and passes on the fix

🤖 Generated with Claude Code

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 0cfe7e65-46b8-4d3d-a70d-9e1736b3fcd2

📥 Commits

Reviewing files that changed from the base of the PR and between ac01b6d and 79338bb.

📒 Files selected for processing (2)

• internal/archdocs/pssg/build/build.go
• internal/archdocs/pssg/build/build_test.go

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix-search-index-rune-truncation

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