refactor: consolidate the duplicated visual-paint heuristics on the SDK's styleInk #3079

Deferred from #3070 (thread).

The repo now carries several independently-maintained "does this element visually paint" heuristics:

• elementPaintsInk / styleInk — packages/sdk/src/adapters/iframe.ts (new in feat(sdk): expose a paint query for transparent-composition hit-testing #3070)
• hasVisualPresence + VISUAL_LEAF_TAGS — packages/studio/src/components/editor/domEditingElement.ts:29
• transparent-background logic duplicated again in studio's captions parser and cli's layout-audit

They already disagree: studio counts box-shadow and audio; the sdk counts the element's own text and per-pixel <img> alpha. That drift is the problem — same-looking predicates answering differently.

Direction: sdk as the shared home, studio delegating. The dependency arrow already points that way (studio → sdk), so studio can import but not the reverse.

But not as one predicate. They answer different questions — "is this element worth showing as an editable leaf" versus "is there ink at this exact point". The per-point half is meaningless for studio, and audio is meaningless for the paint query. The shared core is styleInk plus the transparent-colour parse, with each caller keeping its own policy on top.

Easiest first slice: the transparent-colour test. isTransparentColor in the sdk is a pure function with no policy in it, and it is currently duplicated three times with at least one copy that string-matches known spellings rather than reading the alpha (the bug fixed in #3070) — so the duplicates are likely wrong in the same way.