perf(coverage): emit each HTML page in one awk pass instead of a Bash loop per line #1098

Problem

With the per-line escaping fixed (#1096) the HTML report is 9.5s for 128
files, and 8.1s of that is generate_file_html — 63 ms per page.

Where that goes, measured on Bash 3.2 arm64 over 128 files / 22,405 source
lines:

There are no forks left in that loop — it is ~10 echos of markup per source
line, 22,405 times, plus the string building for the hit badge and tooltip.
Bash is simply the wrong tool for emitting 7 MB of HTML.

Proposal

Emit the page body in one awk pass, the shape that took the LCOV report from
3133 ms to 267 ms (#1090). awk already receives everything the loop needs: the
source lines, the propagated hit counts, the executable classification and the
per-line test list.

Also worth taking while there, all per page in report_html itself:
$(pwd), $(get_cached_stats …) and $(path_to_filename …) — 3 forks × 129
pages.

Verification

Byte-identical HTML against a fixed corpus, as in #1096, plus the existing
fork census.