[leshchenko1979]

Summary

box-sizing resolves to border-box when the property is absent — the opposite of the
CSS default. README.md listed both accepted values but never said which one applies when
the property is omitted, so anyone comparing Ogre output against a browser met the
divergence with no warning.

This documents it. The default is unchanged.

• README.md — the box-sizing bullet names the default; a new ### Differences from CSS
  section under Supported CSS Properties states the divergence, the reason (Satori
  compatibility) and the explicit-value escape hatch.
• style/resolve.go — comments at the two places that materialise the default
  (mergeProps baseline, getOr fallback) so a later reader does not "fix" it silently.

Before / after

-- `box-sizing`: content-box, border-box
+- `box-sizing`: content-box, border-box — **defaults to `border-box`, unlike CSS** (see [Differences from CSS](#differences-from-css))

Plus a new section:

Differences from CSS

One default differs from the browser, deliberately:

Property	Ogre default	CSS default	Why
box-sizing	border-box	content-box	Keeps the same input rendering the same here as in Satori, which is the behaviour this engine is built to match.

With border-box, width and height are outer dimensions: padding and border are drawn
inside them. <div style="width:200px;padding:0 50px"> is 200px wide here and 300px wide
in a browser.

Why document rather than change

Reported in #38 with measurements. On a 21-case Chromium comparison suite with one shared
font file, the border-box default accounts for the two largest remaining divergences
(20-boxsizing-default 1.09 %, 17-table-div 1.20 %); every other case is ≤ 0.77 % and
that residual is glyph antialiasing. Flipping the default would move every layout that
relies on it today and would break Satori-style input that omits box-sizing — a
maintainer decision, hence a docs change here and not a behaviour change.

The explicit-value half is a separate gap, tracked in #26: box-sizing: content-box is
parsed but not yet read by the layout package (grep -r BoxSizing layout/ is empty), so
the escape hatch the README now mentions starts working when that lands. This PR does not
stack on it.

Verification

• go build ./cmd/ogre — exit 0.
• git diff origin/main --stat — README.md, style/resolve.go only; the Go diff is
  three added comment lines, no code change.
• The box-sizing defaults to border-box (CSS default is content-box) and the default is undocumented #38 probe <div style="width:200px;padding:0 50px;background:#f00">x</div> renders a
  byte-identical SVG from this branch and from main (md5 87b96f18fe2173e3135318763215b9c8),
  200 px wide in both.
• style/resolve_test.go already asserts the border-box default for an unstyled div;
  untouched, and it is the lock on the decision.

Refs #38