fix(coverage): a brace inside a string or comment swallows every later function in the file

[Chemaclass]

Problem

extract_functions decides where a function ends by counting { and } on
each line, with no regard for quoting or comments. One unbalanced brace inside
a string or a comment therefore never balances out, and the function it is in
absorbs the rest of the file.

src/coverage/lines.sh is the worst case in this repo: 11 declarations, 1
extracted record. The running balance never returns to zero after three
lines that are not code at all:

 21 bal=1 function bashunit::coverage::is_executable_line() {
 60 bal=2 # Function declarations: `[function ]name()` with an optional trailing `{`.
 72 bal=3 *'{')
 73 bal=4 fn_rest="${fn_rest%'{'}"
 122 bal=3 }
 124 bal=4 function bashunit::coverage::get_executable_lines() { <- never recorded

Across src/: 13 files lose functions, 80 of 760 declarations missing.
Every one of them collapses to a single record spanning most of the file.

Impact

• LCOV FN/FNDA name the wrong span, and FNF/FNH are undercounted, so
  any consumer that reads function coverage (Coveralls, Codecov, SonarQube)
  gets wrong data
• The HTML and text function views show one huge function instead of the file
• A function whose span is wrong takes another function's hits as its own

Fix

Count braces only outside single quotes, double quotes and comments. A #
starts a comment only where bash starts one (start of line or after
whitespace), so ${x#foo} and a#b keep their braces.

Verifying it

Bash itself is the oracle: shopt -s extdebug + declare -F <name> reports
the file and start line of every function it sourced from src/, which pins
the extracted starts against the real ones (in a subshell — extdebug in the
parent clobbers caller state, #808).

[Chemaclass]

Oracle run, for the acceptance bar. Sourcing src/ and reading declare -F
under extdebug gives 697 real functions with their real start lines. Against
that, today's extractor is:

oracle_functions=697 missing_or_wrong_start=19 spurious=2

src/coverage/functions.sh missing=0 spurious=1 END|90
src/coverage/lines.sh missing=10 spurious=0
src/coverage/rules_awk.sh missing=0 spurious=1 END|130
src/helper/discovery.sh missing=9 spurious=0

Two failure modes, both from the same blind spot:

• missing — an unbalanced brace in a comment or a quoted string keeps the
  first function open, so every later one in the file is lost
• spurious — END { inside the single-quoted awk program in
  rules_awk.sh is read as a function declaration named END, because a
  string that spans lines is not tracked as a string

So the counter needs quote state that survives across lines, not only within
one, and heredoc bodies have to be skipped for the same reason.

missing_or_wrong_start=0 spurious=0 on this oracle is the test.

[Chemaclass]

Done in #1087 (merged).