add GfmListblock for GFM lists

GfmListblock captures an entire list block atomically with one
addSpecialPattern match, then walks the captured text in handle()
grouping lines into items. Each item's

add GfmListblock for GFM lists

GfmListblock captures an entire list block atomically with one
addSpecialPattern match, then walks the captured text in handle()
grouping lines into items. Each item's body is dedented to its
content column and parsed by ModeRegistry::getSubParser() so
block content (paragraphs, fenced code, blockquotes, plugin
blocks) works inside items uniformly. Sub-parsed calls are wrapped
in a Nest call before they reach the outer handler, matching the
Footnote pattern: the main handler's Block rewriter treats nest
as opaque and the renderer base class unwraps it transparently,
so multi-paragraph items don't get double-wrapped in <p>.

Marker syntax: -, *, + (unordered) or 1-9 digits followed by
. or ) (ordered). Indentation is a 2-space-multiple step starting
at 0; depth = (indent / 2) + 1, odd indents round down, tabs become
two spaces. The first ordered item's number drives the start
attribute on <ol> via the listo_open $start parameter.

GfmLists subclasses AbstractListsRewriter with the GFM marker
parser; the state machine on the base class is shared with DW Lists.

GfmListblock loads only when $conf['syntax'] is markdown or md+dw.
Under those settings the DW Listblock is suppressed because the two
list models conflict — DW's mandatory 2-space indent rule vs GFM's
zero-indent top-level rule, and -/*/+ markers shared. Plugins that
relied on Listblock loading under md+dw will see it absent there.

Sub-parser exclusion set: CATEGORY_BASEONLY (no Header inside list
items) and gfm_listblock itself (defensive guard against re-entry
on pathological inputs; nested lists are handled by the outer
pattern, not by re-entry).

Tests cover marker variants, ordered start numbers, nested lists at
two and three levels, inline formatting inside items, marker-
character switches keeping one list, type switches splitting the
list, fenced code inside items, multi-paragraph (loose) items, and
two regressions on blank-line tolerance inside the captured block.
SpecCompatRenderer learns to render the list call sequence, and
spec.txt tests for digit/marker-width/lazy-continuation behavior
that GfmListblock deliberately doesn't implement are documented in
gfm-spec/skip.php with the per-bucket reasons (A-F).

Drops two now-obsolete entries from skip.php (image escapes that
land via earlier GfmLink/GfmMedia work) and inlines the Setext
explanation that previously pointed at SPEC.md. Replaces the
SPEC.md reference in GfmEmphasisTest with the inline reason.

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