fix(compile): use rune slice to truncate history summary — avoids UTF-8 boundary corruption

[mpeter]

Root cause

buildCompiledArticle in tools/compile.go truncates learning content for the history table using a byte slice:

summary = summary[:80] + "..."

In Go, string[i:j] slices by bytes, not characters. When a multibyte UTF-8 character (e.g. em-dash U+2014, encoded as 0xE2 0x80 0x94) spans the byte-80 boundary, the slice cuts the character mid-encoding. Appending "..." then places 0x2E (period) as the third byte of the incomplete sequence, producing 0xE2 0x80 0x2E — invalid UTF-8.

Fix

Convert to []rune before slicing, then back to string:

if runes := []rune(summary); len(runes) > 80 {
    summary = string(runes[:80]) + "..."
}

Reproducer

Any learning whose information field contains a multibyte character (em-dash, curly quote, arrows, CJK, etc.) where the UTF-8 encoding spans byte offset 80. Observed in production on the process-git compiled article — every compile produced 0xE2 0x80 0x2E causing UnicodeDecodeError in Python consumers of the article.

Testing

go test ./tools/...  # 438 tests pass

[yvonnedevlinrh]

Good catch, the byte-slicing bug is a subtle one and this is exactly the right fix. The writeup in the PR description is really helpful too, appreciate the detail.

Approved.

I'll create a follow-up issue to add a regression test covering multibyte truncation, just so we have coverage if this area changes down the line.

[jflowers]

Good fix — the rune-slice approach is the correct solution for UTF-8 safe truncation.

Two notes for next time:

1. Use an OpenSpec for bug fixes. Even single-file fixes benefit from the lightweight openspec/changes/ workflow — it creates a traceable record (proposal + tasks), and the tasks artifact is where the regression test requirement (TC-006) gets captured before implementation begins. Running /opsx-propose takes a couple of minutes and would have caught the missing test gap during planning rather than during review.

2. Regression test (TC-006). Convention pack rule TC-006 [MUST]: bug fixes must include a regression test. @yvonnedevlinrh has committed to a follow-up issue for this — please ensure it's filed and linked.

This review was generated by /review-pr (AI-assisted).

[yvonnedevlinrh]

Created #81 for the following on issue of adding in regression testing