parserutils: allow callers to override the parse syntax, key cache on it

With the registry now carrying the flavour as a parameter, expose that
to callers: p_get_instructions(), p_cached_instruction

parserutils: allow callers to override the parse syntax, key cache on it

With the registry now carrying the flavour as a parameter, expose that
to callers: p_get_instructions(), p_cached_instructions() and
p_cached_output() gain an optional $syntax argument. null (the default)
means "use the configured $conf['syntax']", preserving behaviour for
every existing call site; an explicit flavour parses under it regardless
of the wiki's configured preference.

This is what lets bundled assets render deterministically — e.g. a
plugin forcing 'dw' on a document whose configured syntax is 'md'.

Because that case renders the same file under two flavours within one
request, key both the in-request memo (the $run map) and the on-disk
cache on $syntax so the two do not collide. The syntax enters the cache
key only when passed explicitly (non-null); when null the key is
unchanged, so existing caches are untouched. Plumbed through new
optional $syntax args on CacheParser / CacheInstructions, appended to
the key string.

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