add GfmEscape for GFM backslash escapes

Implements GFM §6.1 backslash-escape handling. GfmEscape is a sort-5
inline mode in CATEGORY_SUBSTITION that claims `\X` for any escapable
ASCII punctuation c

add GfmEscape for GFM backslash escapes

Implements GFM §6.1 backslash-escape handling. GfmEscape is a sort-5
inline mode in CATEGORY_SUBSTITION that claims `\X` for any escapable
ASCII punctuation char before competing delimiters can match. The
shared character class lives on Helpers\Escape so the lexer pattern
and the post-hoc unescape stay in lockstep.

Whole-span captures (GfmCode info string, GfmLink label/URL) bypass
the lexer; those modes call Escape::unescapeBackslashes() on the
relevant slot. GfmLink skips the unescape when the URL classifies as
a windowssharelink so the leading \\host survives intact.

GfmTable cells get a separate per-cell `\|` to `|` pass in the
rewriter to honour the tables-extension rule that pipes always
unescape, even inside code spans where standard §6.1 escapes don't
fire.

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add GfmTable for GFM tables

Implements the GFM pipe-table extension as a CONTAINER mode at sort 55,
one below DW Table at 60. A lookahead-validated entry pattern asserts a
header line plus a `:?-+:?

add GfmTable for GFM tables

Implements the GFM pipe-table extension as a CONTAINER mode at sort 55,
one below DW Table at 60. A lookahead-validated entry pattern asserts a
header line plus a `:?-+:?` delimiter row before consuming any input, so
non-table paragraphs containing pipes flow through unchanged. Cells are
inline-only per spec.

Handler\GfmTable rewrites the flat token stream into the canonical
table_open / tablethead_* / tabletbody_* / table_close sequence, deriving
per-column alignment from the delimiter row, padding short body rows
(spec 202), truncating long ones (spec 204), and falling back to a single
cdata when the column count mismatches (spec 203).

`tabletbody_open` / `tabletbody_close` are emitted for the first time;
they are part of the base renderer API but DW Table never used them.
Added to Block's blockOpen / blockClose lists alongside `tabletfoot_*`
for symmetry. SpecCompatRenderer gains minimal table-element overrides
so spec roundtrip output matches GFM's `<table><thead><tr><th>` shape
without DW's wrapper div, row/col counter classes, or align-as-class.

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