feat: add algorithmic size and font scales

Add :scales config to tokens.edn with two generated scales:
- Size (linear): --size-1 through --size-16, base 0.25rem × N
- Font (geometric): --font-xs through --font-3xl, ratio 1.25

gen.clj computes values and emits them into :root only (not dark
theme blocks). button.css now uses scale vars instead of raw rem.
Change base or ratio in tokens.edn to recompute the entire scale.

AGENTS.md

@@ -149,6 +149,8 @@ No changes needed in `gen.clj` — it collects all `src/ui/*.css` files automati
 CSS conventions:
 - Utility-style flat classes: `.component`, `.component-variant`, `.component-size`
 - Use `var(--token-name)` for all colors, borders, shadows, radii
+- Use `var(--size-N)` for spacing/padding/gap/line-height — no raw `rem` values
+- Use `var(--font-*)` for font-size — no raw `rem` values
 - Include hover/focus/disabled states
 - Keep specificity flat — no nesting beyond `:hover:not(:disabled)`
 
@@ -204,9 +206,44 @@ Semantic + scale tokens in `src/theme/tokens.edn`:
 - **Shadows**: `shadow-0/1/2/3` (increasing elevation)
 - **Radii**: `radius-sm/md/lg`
 
+### Algorithmic scales
+
+Defined in `:scales` in `tokens.edn`. Generated into `:root` only (not duplicated in dark theme blocks).
+
+**Size scale** — linear: `--size-N = base × N`
+
+```edn
+:size {:base 0.25 :unit "rem" :steps 16}
+```
+
+Produces `--size-1: 0.25rem` through `--size-16: 4rem`. Use for all spacing, padding, gap, and line-height values.
+
+**Font scale** — geometric: `--font-{label} = base × ratio^power`
+
+```edn
+:font {:base 1 :unit "rem" :ratio 1.25
+       :steps [[-2 "xs"] [-1 "sm"] [0 "base"] [1 "md"] [2 "lg"] [3 "xl"] [4 "2xl"] [5 "3xl"]]}
+```
+
+Produces `--font-xs: 0.64rem` through `--font-3xl: 3.052rem`. Use for all font-size values.
+
+To adjust the entire scale, change `base` or `ratio` — all values recompute on `bb build-theme`.
+
+**Usage in component CSS:**
+
+```css
+.btn {
+  padding: var(--size-2) var(--size-4);
+  font-size: var(--font-sm);
+  line-height: var(--size-5);
+}
+```
+
+**Rule: never use raw `rem` values in component CSS** — always reference a scale variable.
+
 ### Adding tokens
 
-Add to both `:tokens` (light) and `:themes > :dark` in `tokens.edn`. They must have the same keys — the `tokens-roundtrip-test` enforces this.
+Add to both `:tokens` (light) and `:themes > :dark` in `tokens.edn`. They must have the same keys — the `tokens-roundtrip-test` enforces this. Scale config (`:scales`) is theme-independent and lives at the top level.
 
 ### Dark mode
 

src/theme/tokens.edn

@@ -1,4 +1,16 @@
-{:tokens
+{:scales
+ {:size {:base 0.25 :unit "rem" :steps 16}
+  :font {:base 1 :unit "rem" :ratio 1.25
+         :steps [[-2 "xs"]
+                 [-1 "sm"]
+                 [0  "base"]
+                 [1  "md"]
+                 [2  "lg"]
+                 [3  "xl"]
+                 [4  "2xl"]
+                 [5  "3xl"]]}}
+
+ :tokens
  {:bg-0      "#ffffff"
   :bg-1      "#f5f5f5"
   :bg-2      "#e8e8e8"

src/ui/button.css

@@ -3,10 +3,10 @@
   align-items: center;
   justify-content: center;
   gap: 0.5em;
-  padding: 0.5rem 1rem;
+  padding: var(--size-2) var(--size-4);
-  font-size: 0.875rem;
+  font-size: var(--font-sm);
   font-weight: 500;
-  line-height: 1.25rem;
+  line-height: var(--size-5);
   border: none;
   border-radius: var(--radius-md);
   cursor: pointer;
@@ -48,15 +48,15 @@
 }
 
 .btn-sm {
-  padding: 0.25rem 0.5rem;
+  padding: var(--size-1) var(--size-2);
-  font-size: 0.75rem;
+  font-size: var(--font-xs);
-  line-height: 1rem;
+  line-height: var(--size-4);
 }
 
 .btn-lg {
-  padding: 0.75rem 1.5rem;
+  padding: var(--size-3) var(--size-6);
-  font-size: 1rem;
+  font-size: var(--font-base);
-  line-height: 1.5rem;
+  line-height: var(--size-6);
 }
 
 .btn:disabled {

src/ui/css/gen.clj

@@ -21,6 +21,40 @@
        (map (fn [[k v]] (str "  " (token->css-var k) ": " v ";")))
        (str/join "\n")))
 
+(defn format-number
+  "Format a number: strip trailing zeros, max 3 decimal places."
+  [n]
+  (let [s (format "%.3f" (double n))]
+    (-> s
+        (str/replace #"0+$" "")
+        (str/replace #"\.$" ""))))
+
+(defn generate-size-scale
+  "Generate linear size scale: --size-N = base * N."
+  [{:keys [base unit steps]}]
+  (->> (range 1 (inc steps))
+       (map (fn [n]
+              (str "  --size-" n ": " (format-number (* base n)) unit ";")))
+       (str/join "\n")))
+
+(defn generate-font-scale
+  "Generate geometric font scale: --font-{name} = base * ratio^power."
+  [{:keys [base unit ratio steps]}]
+  (->> steps
+       (map (fn [[power label]]
+              (str "  --font-" label ": "
+                   (format-number (* base (Math/pow ratio power)))
+                   unit ";")))
+       (str/join "\n")))
+
+(defn generate-scales
+  "Generate CSS variable declarations for all scales."
+  [scales]
+  (str/join "\n"
+    (cond-> []
+      (:size scales) (conj (generate-size-scale (:size scales)))
+      (:font scales) (conj (generate-font-scale (:font scales))))))
+
 (defn base-css
   "Generate base body/reset styles."
   []
@@ -42,9 +76,12 @@
 
 (defn generate-css
   "Generate the full CSS output from parsed token data."
-  [{:keys [tokens themes]}]
+  [{:keys [tokens themes scales]}]
   (let [dark-tokens (get themes :dark)
-        root-block  (str ":root {\n" (tokens->css-block tokens) "\n}")
+        scale-vars  (when scales (generate-scales scales))
+        root-block  (str ":root {\n" (tokens->css-block tokens)
+                         (when scale-vars (str "\n" scale-vars))
+                         "\n}")
         dark-attr   (str "[data-theme=\"dark\"] {\n" (tokens->css-block dark-tokens) "\n}")
         dark-media  (str "@media (prefers-color-scheme: dark) {\n"
                          "  :root:not([data-theme=\"light\"]) {\n"

test/ui/theme_test.clj

@@ -40,6 +40,21 @@
     (testing "dark media query excludes explicit light theme"
       (is (str/includes? css ":root:not([data-theme=\"light\"])")))
 
+    (testing "contains size scale variables"
+      (doseq [n (range 1 17)]
+        (is (str/includes? css (str "--size-" n ":"))
+            (str "Missing size-" n))))
+
+    (testing "contains font scale variables"
+      (doseq [label ["xs" "sm" "base" "md" "lg" "xl" "2xl" "3xl"]]
+        (is (str/includes? css (str "--font-" label ":"))
+            (str "Missing font-" label))))
+
+    (testing "scales only in :root, not in dark theme blocks"
+      (let [dark-block (second (str/split css #"\[data-theme=\"dark\"\]"))]
+        (is (not (str/includes? dark-block "--size-1:")))
+        (is (not (str/includes? dark-block "--font-base:")))))
+
     (testing "contains button component CSS"
       (is (str/includes? css ".btn {"))
       (is (str/includes? css ".btn-primary {"))