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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rename syntax flavors 'dokuwiki' / 'markdown' to 'dw' / 'md'

Symmetry with the existing 'dw+md' / 'md+dw' setting values.

add GfmLinebreak for GFM hard line breaks

Two-or-more trailing spaces, or a single backslash, immediately before
a non-final newline render as a `<br/>`. Both delimiter forms share a
single SUBSTITI

add GfmLinebreak for GFM hard line breaks

Two-or-more trailing spaces, or a single backslash, immediately before
a non-final newline render as a `<br/>`. Both delimiter forms share a
single SUBSTITION mode at sort 140, loaded under any MD-active syntax
(markdown, dw+md, md+dw); pure dokuwiki is unaffected.

Reuses the existing `linebreak` handler call and renderer; no new
instructions or renderer changes. SpecCompatRenderer overrides
linebreak() to emit the spec's `<br />` shape. Examples 662, 663
(line break inside a raw HTML tag) are skipped — raw HTML is not
passed through by default.

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