Promote crop-bleed to a universal frame-overflow facet

Decision

Adopt frame-overflow: clip | bleed as a universal frame facet, with a policy/signal split:

• Policy — frame-overflow resolves (build-time, guest-agnostic) to data-frame-overflow on the host's [data-section="media"] frame target.
• Signal — the guest sets data-overflowing on itself at runtime when it measures its content as wider than the frame.
• A single shared CSS rule bleeds [data-frame-overflow="bleed"] > [data-overflowing] on a narrow viewport. sandbox is the first consumer; the held bleed="crop" PR is reworked, not merged as-is.

Sub-decisions:

• Host-set is canonical. The facet is authored on the host/slot ({% hero frame-overflow="bleed" %}), matching the rest of frame-*. The guest-self route (giving a guest frameTarget: 'self' so it can carry frame-overflow directly) is deferred — additive and non-breaking, but it also pulls the rest of the frame chrome onto the guest, so it should be a deliberate later step if host-set ergonomics prove awkward.
• Hard build warning on clip hosts. frame-overflow="bleed" is inert on a clip host (guestFit: 'clip') because the media well's overflow: hidden clips the over-width. Rather than a silent no-op, the engine emits a hard warning naming the rune and pointing to a bleed host — same precedent as warnFrameNoTarget and the SPEC-090 interaction-posture warnings.
• Name: frame-overflow (values clip | bleed). Parallels CSS overflow; kept distinct from frame-displace-mode's peek|bleed (which moves the whole guest) and from showcase's deprecated bleed alias.
• v1 is narrow; direction and side-by-side are deferred. v1 ships the collapsed/stacked, inline-end case only. Two extensions are designed but out of v1:
  • Bleed direction reuses frame-anchor. When built, frame-anchor's inline keyword chooses the anchored edge (start → bleed end, end → bleed start, center → bleed both), so frame-anchor means one thing — "the focal point kept" — across object-fit crop and overflow bleed. No new direction vocabulary on frame-overflow.
  • Side-by-side bleeds the outer edge, forced by media-position; deferred for its heavier geometry (escaping the grid track + page margin, interacting with contentMeasure). The author surface frame-overflow="bleed" is stable across both — the engine derives direction and edge; the attribute never changes — so the narrow v1 does not paint us into a corner.
• The bleed boundary is layout-owned, never the raw viewport. A bleed extends to the current layout's outermost content track — the viewport in the default layout, but only the wide track (inside the sidebar/TOC) in the docs layout, which has no viewport track. It is expressed as a layout-set inset (--rf-bleed-room-*, default page gutter), the same boundary width="full" honours. v1 adopts the variable seam even though its collapsed trigger (≤640px, where docs chrome is already hidden) does not expose the collision — so the deferred desktop / side-by-side / docs cases inherit a correct boundary.