fix(reports): escape test titles in the HTML report

[Chemaclass]

🤔 Background

Related #1249

src/reports/html.sh had no escaping at all. A test title is user text — bashunit::set_test_title takes anything, and a data provider interpolates values into it — and it went straight into the markup:

<td><script>alert(1)</script> & <b>bold</b></td>

A < corrupts the table, a bare & is invalid entity syntax, and a script tag runs in whoever opens the report — for a CI artifact, a browser. Every other writer escapes (JUnit, JSON, Markdown, GHA); this was the only one that did not.

Same cause, second defect: rows were joined into a temp file with | and split back on it, so a title carrying one shifted every column and emitted class="after" — a CSS class matching no rule, so the row silently lost its status colour.

💡 Changes

• Separate the fields with US (0x1f) instead of |
• Escape &, <, > and " in one awk pass over the collected rows — one fork per report, not per test
• Five tests: markup neutralised, & escaped, columns stay aligned, the title still reads as written, and a plain title is untouched so the escaping is not over-eager

awk rather than ${var//&/&} for the reason already recorded in #1096: a bare & in a bash replacement means "the matched text" from 5.2 on while staying literal on 3.2. The same rule applies to gsub, hence \\&, and the separator reaches awk as a byte rather than \x1f, which is not POSIX awk (#1098). This is the approach src/coverage/html_file.sh already uses for the coverage pages.