fix (parsing): avoid newline loss on GFM section editing

The GFM header parsing returned a byte position pointing at the newline
before the actual header resulting in the observed newline eatings as

fix (parsing): avoid newline loss on GFM section editing

The GFM header parsing returned a byte position pointing at the newline
before the actual header resulting in the observed newline eatings as
reported in https://github.com/dokuwiki/dokuwiki/pull/4636#issuecomment-4491970909

Additionally this fixes an oddity of DW header parsing which
accidentally allowed text on the line before the opening = chars.
Whitespace is still allowed.

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

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

add GfmHeader for ATX headings (`# text` through `###### text`)

Opener must sit at column 0. GFM tolerates 0-3 spaces before the `#`
but that collides with DokuWiki's 2-space-indent preformatted blo

add GfmHeader for ATX headings (`# text` through `###### text`)

Opener must sit at column 0. GFM tolerates 0-3 spaces before the `#`
but that collides with DokuWiki's 2-space-indent preformatted block,
so the tolerance is dropped rather than plumbed across modes.

Widen the XHTML renderer's section-node tracker from 5 slots to 6 so
h6 doesn't hit "Undefined array key 5". Extend GfmSpecTest's HTML
normalizer to strip DokuWiki's section-div wrappers, section-edit
comments, and header id/class attributes so heading spec examples
can validate semantic correctness.

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