declaration: read the whole shape-outside grammar

[samoht]

Stacked on #298 (chain: #295 → #297 → #298 → #299). Review the last two commits; the rest is the base.

read_shape_outside looked at the first component only and knew three arms of the grammar — none, circle() and a non-empty inset(), plus the CSS-wide keywords. Everything else was rejected and the declaration dropped:

shape-outside: margin-box                 dropped
shape-outside: circle(50%) content-box    dropped
shape-outside: url(shape.png)             dropped
shape-outside: ellipse(closest-side ...)  dropped

CSS Shapes 1 §2 defines it as none | [<basic-shape> || <shape-box>] | <image>. The reader now takes that whole grammar, reusing read_clip_path for the <basic-shape> || <box> pair and read_background_image for <image>, with the two points where Shapes 1 differs from clip-path checked explicitly:

• the box is a <shape-box> (§2.2), so the three SVG boxes <geometry-box> adds — fill-box, stroke-box, view-box — are not valid here;
• everything outside <basic-shape> is an alternative to the pair, not a member of it.

The value stays raw text (Shape_outside : string property); typing it would mean a sum of the clip_path shapes plus the whole background_image type, so the reader validates the grammar and hands the source back verbatim. One test_optimize normalization pair moved from shape-outside to clip-path for that reason — a raw-text value is never normalized, so it could not demonstrate what that test asserts.

The vendor-prefixed reader gap filed alongside this one is not included; its pin lives on declaration-vendor-readers and is still failing.