fix(capture): warn 300–799px PNG floor; hard-block only below 300

Split FLOOR_WARN_PX=800 / FLOOR_BLOCK_PX=300. Warn-band still encodes and
includes the photo (Photos-ready + batchApply ok+warn). Block excludes
siblings and clean cal. CAPTURE-105 / CAPTURE.md / PHOTOS.md updated.

frontend/src/measure/measure.ts

@@ -92,7 +92,7 @@ export function measure(
   lengthEighths: Eighths, thicknessEighths: Eighths,
   slabPx: number,
   tape?: TapeAxis | null,
-): Omit<Measures, 'inventoryPng'> & { sourceBelowFloor: false } {
+): Omit<Measures, 'inventoryPng'> & { sourceBelowBlock: false; sourceBelowWarn: false } {
   const lengthIn = lengthEighths / 8;
   let start: [number, number];
   let ux: number;
@@ -132,6 +132,7 @@ export function measure(
     widths,
     sqft,
     bdft,
-    sourceBelowFloor: false,
+    sourceBelowBlock: false,
+    sourceBelowWarn: false,
   };
 }