html/layout: optional flush (outline) list-marker alignment

[carlos7ags]

Follow-up to #358/#360. Adds an opt-in mode that draws every nested list
marker in the container's left column (all numbers share one left edge) while
the body text still cascades per level — the "outline" style common in legal
documents — as an alternative to the per-level marker indent that is now the
default.

Comparison

Same Master Services Agreement (examples/legal-numbering), rendered both ways:

Nested (default)                 Flush (FlushListMarkers: true)
1.   Definitions.                1.   Definitions.
  1.1.  "Agreement" ...            1.1.  "Agreement" ...
  1.2.  "Services" ...             1.2.  "Services" ...
    1.2.1.  Each statement ...       1.2.1.    Each statement ...
    1.2.2.  In the event ...         1.2.2.    In the event ...
2.   Term and Termination.       2.   Term and Termination.

In nested, 1.2.1./1.2.2. indent under "Services"; in flush they stay in the
left column and the clause bodies hang. Reproduce locally by rendering the
legal-numbering example HTML with &fhtml.Options{FlushListMarkers: true}.

API

Document-wide, off by default:

doc.AddHTML(html, &fhtml.Options{FlushListMarkers: true})

Underneath, layout.List.SetMarkerFlush(bool) toggles it; sub-lists inherit
the value. Only the left-to-right outside path is affected; RTL and
list-style-position: inside are unchanged. Default (nested) output is
byte-identical.

Scope

Kept intentionally minimal: a single document-wide option, no new CSS
property. If per-<ol> control within one document is needed later, a
(non-standard) CSS property can layer on top of the same SetMarkerFlush
plumbing.

Tests

TestNestedMarkerFlush (layout: all markers share x=0 while bodies cascade) and
TestFlushListMarkersOption (the Options toggle reaches the layout and changes
the render while numbering stays intact). Full module suite passes.