Add per-link configuration attributes to L<> markup code

[zag]

Problem

L<> currently accepts only URL and display text. No way to specify per-link metadata like "open in new tab" or "this is a download".

=config L<> sets defaults for entire document, but often need different behavior for specific links.

Solution

Extend L<> syntax to accept attributes after URL:

L<text|url :attr<value>>

Boundary: first space + colon starts config section. URLs don't contain this sequence, so parsing is unambiguous.

Syntax

L<API docs|https://api.example.com :new>
L<Download SDK|file:sdk.tar.gz :download>
L<French docs|doc:Guide :lang<fr> :title('Version française')>

Attributes

Four core attributes:

Attr	Type	What it does
:new	bool	open in new context (tab/window/pane)
:title	string	tooltip / advisory text
:lang	string	language of target
:download	bool/string	mark as download; string = filename

Precedence

Inline overrides =config:

=config L<> :new

L<external|https://x.com>           # gets :new from config
L<internal|doc:Local :!new>         # override: no new context

Edge cases

File paths with spaces must be quoted:

L<file:'my doc.pdf' :download>      # correct
L<file:my doc.pdf :download>        # wrong — parser error

URL path params (:id) distinguished by no preceding space:

L<https://api.com/users/:id :new>
                       ^^^^ ^^^^
                       URL  attr

Notes

No breaking changes . Existing L<> without attributes works exactly as before.
Same syntax applies to W<> contextual backlinks.