Figma transformer: responsive emission — consume sibling-groups → emit @media

## Problem

The Figma transformer renders a single fixed-width canvas, not a responsive website. The root wrapper is `.figma-root{position:relative;min-width:100%;width:max-content}` (fixed canvas width → horizontal scroll on narrow viewports) and there are **zero `@media` queries package-wide**. Auto-layout frames map to real flexbox and FILL/HUG sizing is honored, so output is faithful at the design's native width — but it does not reflow.

To date, 35 merged figma PRs are all single-width fidelity/diagnostics work. Responsive **emission** has never been attempted.

## What exists already

Detection of responsive frame sets now lands in `ScenegraphFrameInspector` (PR landing the `cook/figma-selection-responsive` branch):

- `deviceHint()` — classifies a frame `mobile`/`tablet`/`desktop` by name regex + width thresholds (<700 / 700–1100 / ≥1100).
- `siblingGroupKey()` + `responsiveSiblings()` — groups frames that represent one page at multiple widths.
- `normalizedPageName()` / `nameSimilarity()` — strip `desktop`/`mobile`/`copy`/`v2`/`variant` suffixes so siblings match by base name.

This is **detection only**. `device_hint` / `sibling_group_key` / `responsive_siblings` are consumed by nothing — neither `ScenegraphPagePlanner` nor `StaticHtmlEmitter` reads them.

## Scope

1. **PagePlanner** — collapse a responsive sibling-group into a single page with N breakpoint variants (instead of one page per frame).
2. **StaticHtmlEmitter** — diff per-breakpoint styles and emit `@media` blocks; share the existing flex/FILL/HUG machinery across breakpoints rather than re-deriving layout.
3. **Fluid root** — replace `width:max-content` with a centered `max-width` container for FILL roots; clear the `fixed_root_width_count` diagnostic smell.
4. **Per-breakpoint parity** — extend the parity contract so each breakpoint is verified against its corresponding source frame. Responsiveness is unfalsifiable while parity is measured at a single viewport.

## Acceptance

- A Figma file with desktop/tablet/mobile frames of the same page produces one HTML page with `@media` breakpoints that match each source frame.
- Parity report records a pass/fail per breakpoint against its source frame.

Start by tracing the existing detection in ScenegraphFrameInspector into the currently uninvolved ScenegraphPagePlanner and StaticHtmlEmitter. Read the parity contract and its single-viewport measurement to determine how breakpoint-specific source frames are represented. Done means one page from a responsive sibling-group, matching @media output for desktop/tablet/mobile, a fluid FILL root, and a pass/fail parity result per breakpoint.

Written by the indexing model from the issue text.