Bound inferred responsive fallback below desktop-only source frames

[chubes4]

Problem

For a design with one wide desktop frame and no authored responsive variant, the inferred desktop fallback starts one pixel below the source width. Fisiostetic has a single 2095px frame, so the transformer emits @media (max-width:2094px) and rewrites layout across nearly the entire desktop range.

Fresh Fisiostetic trunk evidence:

• Source frame: 2095 × 6975.7265625.
• Generated fallback: @media (max-width:2094px).
• The fallback changes section sizing, flex behavior, gaps, and header geometry at ordinary 1440px desktop width.
• A separate 767px mobile fallback is already emitted.

Owning contract

An inferred fallback should activate at a semantic desktop/tablet boundary derived from layout safety, not at sourceWidth - 1. The exact source canvas width is evidence for proportional scaling, not evidence that every narrower desktop must use repair CSS.

Acceptance

• A lone ultra-wide desktop frame does not create a near-source-width fallback such as 2094px.
• Ordinary desktop widths retain desktop structure with bounded fluid sizing.
• Mobile safety still activates at the mobile breakpoint.
• Header/navigation fallback remains usable without rewriting desktop chrome prematurely.
• Add deterministic breakpoint tests for 1440px and 2095px source frames.

AI assistance

OpenAI GPT-5.6 Sol via OpenCode reproduced the defect on current trunk, traced desktopOnlyResponsiveFallbackMediaBlocks(), and helped formulate the acceptance contract. Chris Huber remains responsible for the resulting change.