docs: code surfaces guide and --ca-code-line-height token (integer px line-height) #551

Atlas app: an editor's caret and active-line highlight drifted apart. The lib ships no editor and no line highlight, but its code surfaces (.ca-code pre 0.8125rem/1.7, CodeSnippet font-size caption * 1.6) have fractional line-heights that make any overlay (active line, line numbers, caret) drift.

Plan: token --ca-code-line-height (integer px) used by CodeSnippet and .ca-code/prose pre; guide 'code surfaces: integer line-height, one font stack'; test that the computed line-height is an integer px and the token is applied.