The context block of a rule is rendered seven times in OtlDump

[jakejackson1]

Follow-up to #117, which added the seventh copy.

OtlDump renders the same block of HTML in seven places: the backtrack, input and lookahead
sequences a context rule matches, and the example fragments each contributes.

$html .= '<div class="context">CONTEXT: ';
for ($ff = count($backtrackGlyphs) - 1; $ff >= 0; $ff--) {
    $html .= '<div>Backtrack #' . $ff . ': <span class="unicode">' . $this->formatUniStr($backtrackGlyphs[$ff]) . '</span></div>';
    $exampleB[] = $this->formatEntityFirst($backtrackGlyphs[$ff]);
}
...

where	lookup
OtlDump.php:1140	GSUB 5/1
OtlDump.php:1206	GSUB 5/2
OtlDump.php:1272	GSUB 5/3
OtlDump.php:1351	GSUB 6/1
OtlDump.php:1465	GSUB 6/2
OtlDump.php:1571	GSUB 6/3
OtlDump.php:2857	all six GPOS 7 and 8 formats, in reportGPOSrule()

Six of those are older than #117; the seventh arrived with it, because the GPOS reporters
were written to the shape the GSUB ones already had. reportGPOSrule() is the
generalisation the other six want: it is 1351 line for line, and the class-aware superset
of 1465, which reports class 0 as what it excludes rather than as a list.

What a fix looks like

Lift reportGPOSrule()'s CONTEXT half into a method the seven share. Everything above the
for that walks the lookup records is common; the $class0excl parameter already covers
the one variation, and passing it empty gives the glyph-list form.

What does not share cleanly

The half below it - the $exB/$exL example fragments handed to the nested lookup at
level 2 - is not a drop-in, and the differences are in the output rather than the code:

• GPOS prepends ‍ to each lookahead entry (OtlDump.php:2897) where GSUB appends
  it (OtlDump.php:1399).
• GSUB calls formatEntity() on each input glyph where GPOS reuses the precomputed
  $exampleI, which is formatEntityFirst().

Whether those are two deliberate renderings or one of them is a bug is worth settling
first, because the tests/data/otldump/*.txt fixtures pin both and a shared version has to
pick one.

Safety

tests/data/otldump/*.txt covers every font in tests/data/ttf and pins this markup
exactly, so a share that leaves all 23 byte-identical has not changed the report.