feat(frontend): annotate key numbers with meaning (#18)

Add shared Meaning primitive and pure interpretation helpers for interval,
tyre age, and championship gap columns on the live tower, tyre deg panel,
and championship hub. Thresholds are exported consts (undercut window tied
to PIT_LOSS_SECONDS from #13).

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/src/styles/meaning.css

@@ -0,0 +1,48 @@
+/* Compact value + muted meaning caption (issue #18). */
+
+.meaning {
+  display: inline-flex;
+  flex-direction: column;
+  align-items: inherit;
+  gap: 1px;
+  line-height: 1.2;
+}
+
+.meaning-value {
+  /* inherits table cell mono styling from parent */
+}
+
+.meaning-caption {
+  font-size: 10px;
+  font-family: var(--f-mono);
+  color: var(--text-3);
+  letter-spacing: 0.02em;
+  white-space: nowrap;
+}
+
+.meaning-caption--good {
+  color: var(--green);
+}
+
+.meaning-caption--bad {
+  color: var(--red);
+}
+
+.meaning-caption--warn {
+  color: var(--yellow, #e8c547);
+}
+
+.meaning-caption--neutral {
+  color: var(--text-3);
+}
+
+/* Table cells: right-align caption under numeric values */
+td.r .meaning,
+.champ-td-dim .meaning,
+.champ-td-muted .meaning {
+  align-items: flex-end;
+}
+
+.tyredeg-row .meaning {
+  align-items: flex-start;
+}