feat: closest-pt size selection instead of ordinal slot (#1912)

Add SdCardFontFamilyInfo::pickClosestSize(targetPtSize) and have the
manager and SdCardFontSystem drive size selection from a target point
size derived from the user's font-size enum (SMALL=12, MEDIUM=14,
LARGE=16, EXTRA_LARGE=18) rather than indexing the family's sorted size
list by enum ordinal.

The ordinal-slot approach mis-selects whenever a family doesn't ship the
canonical {12,14,16,18} set: a family with only [10,14,18] would map
SMALL/MEDIUM/LARGE/EXTRA_LARGE to 10/14/18/18 — fine for SMALL but
arbitrary for the rest. Closest-pt always picks the on-disk file nearest
to the user-intended point size, with a deterministic smaller-pt
tie-break.

No change for canonical-sized families.

## Summary

* **What is the goal of this PR?** (e.g., Implements the new feature for
file uploading.)
* **What changes are included?**

## Additional Context

* Add any other information that might be helpful for the reviewer
(e.g., performance implications, potential risks,
  specific areas to focus on).

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src/SdCardFontSystem.cpp

@@ -5,10 +5,15 @@
 
 #include "CrossPointSettings.h"
 
-static uint8_t fontSizeEnumFromSettings() {
+// Map fontSize enum (SMALL=0, MEDIUM=1, LARGE=2, EXTRA_LARGE=3) to the point
+// sizes shipped with the built-in fonts. Used to drive closest-pt selection
+// in the SD card font registry (see SdCardFontFamilyInfo::pickClosestSize).
+static constexpr uint8_t FONT_SIZE_TO_PT[CrossPointSettings::FONT_SIZE_COUNT] = {12, 14, 16, 18};
+
+static uint8_t targetPtSizeFromSettings() {
   uint8_t e = SETTINGS.fontSize;
   if (e >= CrossPointSettings::FONT_SIZE_COUNT) e = 1;  // default to MEDIUM
-  return e;
+  return FONT_SIZE_TO_PT[e];
 }
 
 void SdCardFontSystem::begin(GfxRenderer& renderer) {
@@ -25,7 +30,7 @@ void SdCardFontSystem::begin(GfxRenderer& renderer) {
   if (SETTINGS.sdFontFamilyName[0] != '\0') {
     const auto* family = registry_.findFamily(SETTINGS.sdFontFamilyName);
     if (family) {
-      if (manager_.loadFamily(*family, renderer, fontSizeEnumFromSettings())) {
+      if (manager_.loadFamily(*family, renderer, targetPtSizeFromSettings())) {
         LOG_DBG("SDFS", "Loaded SD card font family: %s", SETTINGS.sdFontFamilyName);
       } else {
         LOG_ERR("SDFS", "Failed to load SD font family: %s (clearing)", SETTINGS.sdFontFamilyName);
@@ -53,7 +58,7 @@ void SdCardFontSystem::ensureLoaded(GfxRenderer& renderer) {
 
   const char* wantedFamily = SETTINGS.sdFontFamilyName;
   const std::string& currentFamily = manager_.currentFamilyName();
-  const uint8_t sizeEnum = fontSizeEnumFromSettings();
+  const uint8_t targetPt = targetPtSizeFromSettings();
 
   if (wantedFamily[0] == '\0') {
     if (!currentFamily.empty()) {
@@ -62,8 +67,8 @@ void SdCardFontSystem::ensureLoaded(GfxRenderer& renderer) {
     return;
   }
 
-  // Reload if family changed OR if the user-selected size maps to a
-  // different file than what's currently loaded OR if the registry was
+  // Reload if family changed OR if the user-selected size now resolves to a
+  // different on-disk file than what's currently loaded OR if the registry was
   // just rediscovered (file may have been replaced on disk).
   bool familyMatches = (currentFamily == wantedFamily);
   if (familyMatches) {
@@ -74,13 +79,11 @@ void SdCardFontSystem::ensureLoaded(GfxRenderer& renderer) {
       SETTINGS.sdFontFamilyName[0] = '\0';
       return;
     }
-    auto sizes = family->availableSizes();
-    uint8_t idx = sizeEnum;
-    if (idx >= sizes.size()) idx = sizes.size() - 1;
-    uint8_t wantedPt = sizes.empty() ? 0 : sizes[idx];
-    if (!registryWasDirty && wantedPt == manager_.currentPointSize()) return;
-    LOG_DBG("SDFS", "Reloading %s: size %u -> %u (enum %u)%s", wantedFamily, manager_.currentPointSize(), wantedPt,
-            sizeEnum, registryWasDirty ? " [registry dirty]" : "");
+    const auto* best = family->pickClosestSize(targetPt);
+    const uint8_t bestPt = best ? best->pointSize : 0;
+    if (!registryWasDirty && bestPt == manager_.currentPointSize()) return;
+    LOG_DBG("SDFS", "Reloading %s: size %u -> %u (target %u)%s", wantedFamily, manager_.currentPointSize(), bestPt,
+            targetPt, registryWasDirty ? " [registry dirty]" : "");
   }
 
   if (!currentFamily.empty()) {
@@ -89,7 +92,7 @@ void SdCardFontSystem::ensureLoaded(GfxRenderer& renderer) {
 
   const auto* family = registry_.findFamily(wantedFamily);
   if (family) {
-    if (manager_.loadFamily(*family, renderer, sizeEnum)) {
+    if (manager_.loadFamily(*family, renderer, targetPt)) {
       LOG_DBG("SDFS", "Loaded SD font family: %s", wantedFamily);
     } else {
       LOG_ERR("SDFS", "Failed to load SD font family: %s (clearing)", wantedFamily);