Let a weather source describe its own products

A weather source can describe its own products (credit, units, coverage,
legend) through an optional `display` object in each product manifest:
source, credit, units, coverage, info, legend and extent. The weather
layer uses each valid field in the row summary, legend, info text,
coverage navigation, outside-coverage status, layer stats and imagery
credit, and keeps today's built-in value for any field that is absent or
invalid. Units and legend describe one scale, so they apply only as a
valid pair; otherwise both keep their built-in values, and the radar
info heading names the units in use.

Text is single-line and length-capped. It rejects control characters
(U+0000-U+001F, U+007F-U+009F), line and paragraph separators, and
invisible or format characters: soft hyphen (U+00AD), combining grapheme
joiner (U+034F), Arabic letter mark (U+061C), Mongolian vowel separator
(U+180E), zero-width and direction marks (U+200B-U+200F), bidi embedding
and override (U+202A-U+202E), word joiner, invisible operators and bidi
isolates (U+2060-U+2069), variation selectors (U+FE00-U+FE0F), the
byte-order mark (U+FEFF) and tag characters (U+E0000-U+E007F).

Legends need 2-16 stops whose labels are plain decimal numbers or short
text and whose colours are #rrggbb. A number label must read back
exactly as the legend shows it: within the safe integer range, with no
leading zeros and no digits lost to rounding. Extents must be finite, in
range and non-empty. Supplied source and units text is data: the layer
marks it so the panels show it untranslated, while built-in values are
translated as before.

The imagery credit is escaped as plain text and follows a refreshed
manifest even when the shown frame stays the same. A refresh changes
only the credit: the shown frame keeps the snapshot it was staged from,
so a later host switch restages it with its own time and bounds and the
refreshed credit. Manifests without `display`, including the bundled
nowCOAST ones, render exactly as before.

src/i18n/locales/it.json

@@ -1214,6 +1214,8 @@
   "Contiguous US · gaps ≠ no rain": "USA contigui · lacune ≠ assenza di pioggia",
   "Americas + Pacific · not individual strikes": "Americhe + Pacifico · non singoli fulmini",
   "Color: strikes/km²/min ×10³": "Colore: fulmini/km²/min ×10³",
+  "Color: {units}": "Colore: {units}",
+  "RADAR · {units}": "RADAR · {units}",
   "60°S–60°N · typically 2–3 h delayed": "60°S–60°N · di solito 2–3 h di ritardo",
   "North America · infrared imagery": "Nord America · immagini all'infrarosso",
   "Reduced motion · history playback unavailable": "Movimento ridotto · riproduzione dello storico non disponibile",