Parsing: make parse syntax a per-parse value, drop ModeInterface

The active parse's syntax flavour is a per-parse question, not process-
global state: within a single request a plugin can render bun

Parsing: make parse syntax a per-parse value, drop ModeInterface

The active parse's syntax flavour is a per-parse question, not process-
global state: within a single request a plugin can render bundled
DokuWiki-syntax text inside an otherwise-Markdown page. Yet ModeRegistry
was a singleton that read $conf['syntax'] and the $PARSER_MODES global,
and every mode reached it through ModeRegistry::getInstance() — so the
flavour lived in shared mutable state that two parses in one request
would fight over.

Make the registry a short-lived value instead:

- ModeRegistry is constructed once per parse with an explicit $syntax
and injected into Parser, Handler and every mode. getSyntax() /
isDwPreferred() / isMdPreferred() consult $this->syntax; the
DOKU_UNITTEST-gated mode-list cache hack is gone (each registry is
fresh, nothing to invalidate).
- p_get_instructions() is now the single place in the pipeline where
$conf['syntax'] is read; from there the flavour travels as a
parameter. No code under inc/Parsing/ reads $conf['syntax'] directly
anymore — the five syntax-reading modes (Preformatted, GfmHr,
GfmEscape, Externallink, GfmQuote) route through $this->registry.

Keep the two concepts apart, as documented in the ModeRegistry and
AbstractMode docblocks: the user's configured *preference* stays in
$conf['syntax'] for UI code (toolbar, settings), while the active
parse's syntax is a parameter carried by the registry.

$PARSER_MODES is demoted to a deprecated, read-only mirror, published
during loadPluginModes() — third-party syntax plugins (columnlist,
alphalist2, phpwikify, skipentity) and the bundled info plugin read the
global directly, often from their constructors, so the taxonomy must
stay visible there. No core code reads the mirror.

Fold ModeInterface into AbstractMode while here: getSort()/handle() are
abstract, the connect callbacks carry defaults, and the public $Lexer
"FIXME should be done by setter" becomes setLexer()/getLexer() injected
by Parser::addMode() alongside the registry. Nested-content resolution
moves to the allowedCategories()/filterAllowedModes() hooks, resolved
once when the registry is attached.

Tests build their own parser/registry through ParserTestBase::setSyntax()
instead of mutating $conf and calling the removed ModeRegistry::reset().

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GFM modes: follow CATEGORY_SUBSTITION → CATEGORY_SUBSTITUTION rename

Constant was renamed on master (the typo'd 'substition' value is kept,
but the constant name spells it correctly). Update GfmTabl

GFM modes: follow CATEGORY_SUBSTITION → CATEGORY_SUBSTITUTION rename

Constant was renamed on master (the typo'd 'substition' value is kept,
but the constant name spells it correctly). Update GfmTable's use of
the constant, plus stale docblock/comment references in GfmEscape,
GfmHtmlEntity, GfmLinebreak, and GfmLinebreakTest.

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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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