perf(coverage): the HTML report takes a minute because it forks twice per source line

[Chemaclass]

Problem

--coverage-report-html over this repo's src takes 58.7 seconds for 128
files (129 pages) — about 458 ms per page. Every other renderer is now under
300 ms for the same input (#1088, #1090, #1092).

The cause is one line of the per-line loop in html_file.sh:

escaped_line=$(bashunit::coverage::html_escape "$line")

and the helper itself:

printf "%s" "$text" | sed "s/&/\&amp;/g; s/</\&lt;/g; s/>/\&gt;/g"

That is a command substitution and a sed per source line — roughly 22,000
processes for this repo's ~11,000 tracked lines. The tooltip path adds a
basename fork per test that hit a line.

Fix

Escape in Bash into a return slot. Measured on one 413-line file:

	413 lines
printf | sed in $()	877 ms
pure-bash slot	12 ms

Byte-identical over 5,790 real source lines. ${var//…} is quadratic on large
strings, but these are single source lines, which is where it wins.

basename in the tooltip loop becomes ${test_file##*/} for the same reason.

Why it matters

The HTML report is the one a human looks at, and a minute per run is the
difference between using it and not. A 500-file project pays four.

[Chemaclass]

Full fork inventory for the HTML path, so this lands as one fix rather than a
partial one.

Per source line (~11,000 of them here — this is the 58.7s):

• escaped_line=$(bashunit::coverage::html_escape "$line") — the command
  substitution and the sed inside it. Note the $() is half the cost, so
  making the helper pure Bash is not enough on its own; the call site has to
  read a slot.
• basename per test in the tooltip loop, for covered lines with test info.

Per function (~700 here):

• fn_pct=$(bashunit::coverage::calculate_percentage …) in both
  html_file.sh and report_text.sh. perf(coverage): the text report forks a subshell per file just to pick a colour #1092 converted the class beside it to
  a slot but left the percentage forking, which is why that change measured
  4042ms → 3116ms rather than better. Same treatment finishes it.

Per page (129 here, ~6 forks each):

• $(pwd) in the display-path computation
• wc -l <"$file" | tr -d ' ' — two processes for a count the already-loaded
  file_lines array answers
• basename twice for the title and the header, plus once more per row in
  html_index.sh

All of them have a pure-Bash equivalent that is already house style
(${var##*/}, ${#array[@]}, $PWD, return slots).

[Chemaclass]

Done in #1097 (merged). The remaining 8.1s of page emission is #1098.