fix(design): keep below/attention off the brand hue, harden the share-card fonts

Review follow-up on #86.

The blind coral -> brand rename recreated the exact collision this PR set out
to remove: coral had been both the primary CTA and the "below average" signal,
so every negative indicator followed --primary onto iris. Sixteen rules moved
back onto the status ramp — delta chips, trend-down arrows, progress-negative
values, statusBad, chipBad/badgeBad, and the urgent deadline chips.

The Ofsted scale had also lost its worst step, with grade 4 landing on brand
while 1-2 were teal and 3 was amber. It now escalates by weight rather than by
reaching for another hue: a tinted amber chip for "requires improvement", a
solid amber one for "inadequate" (5.1:1 light, 7.7:1 dark). Report-card grade 5
follows the same rule.

globals.css now describes status as valence — teal above/good, amber
below/needs-attention — which is what it has to mean for an urgent deadline,
rather than the narrower "comparison point only" the first draft claimed.

On the share-card fonts: /opengraph-image is prerendered, so the font read
happens in the builder stage where assets/ exists, and the baked PNG ships
inside .next/standalone/.next/server/app/. File tracing independently places
the fonts at .next/standalone/assets, which the existing standalone COPY
carries to /app/assets. So the reported ENOENT doesn't occur — but it depends
on the tracer resolving a runtime join(), and a miss would be a silent 500
rather than a build failure. Declared outputFileTracingIncludes for the route
and made the Dockerfile COPY explicit so neither is left to inference.

Also repointed the immutable Cache-Control rule from the deleted favicon.svg
to app/icon.svg, where it was caching a 404.

Verified: tsc clean, 159/159 tests, clean rebuild prerenders all three image
routes with the fonts present in standalone.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

nextjs-app/app/globals.css

@@ -6,10 +6,14 @@
  * five-bar spread that appears inside every school row.
  *
  * Colour has exactly three jobs and they never borrow each other's hues:
- *   brand   (iris)          interactive + identity. Never means "good".
- *   status  (teal / amber)  above / below the comparison point. Never a CTA.
+ *   brand   (iris)          interactive + identity. Never carries valence.
+ *   status  (teal / amber)  valence — teal is above/good, amber is
+ *                           below/needs-attention. Never a CTA.
  *   phase   (desaturated)   category chips, deliberately subordinate.
  *
+ * Escalate within status by weight, not by reaching for a new hue: a tinted
+ * amber chip for "requires improvement", a solid amber one for "inadequate".
+ *
  * Teal/amber rather than green/red so the above/below signal survives every
  * form of colour blindness — the audience includes parents reading closely.
  */