feat(photos): burn axis-oriented dimension overlay PNG (#63)

Add pure station/axis math and canvas burn-in for a second calibration
inventory PNG (results.overlayPng). Width stations start 6 in from the
EXIF-oriented photo bottom along the approved length axis (6, 18, … ≤
L−6); length graphic sits on axis-right outside the slab. Clean
inventoryPng and batchApply[cal]=null stay unchanged.

frontend/src/capture/session.ts

@@ -9,6 +9,7 @@ import { loadDraft, saveDraft, clearDraft } from '../draft/idb';
 import { measure } from '../measure/measure';
 import { orientedRect } from '../geometry/rect';
 import { encodeInventoryPng, type InventoryInput } from '../png/inventory';
+import { encodeOverlayPng, type EncodeOverlayInput } from '../png/overlay';
 
 const DEBOUNCE_MS = 2000;
 const BUSY_MS = 300;
@@ -52,6 +53,8 @@ export interface SessionOpts {
   draft?: DraftStore;
   /** Injectable only for deterministic tests; production uses encodeInventoryPng. */
   encodeInventory?: (input: InventoryInput) => Promise<{ blob: Blob; sourceBelowFloor: boolean }>;
+  /** Injectable only for deterministic tests; production uses encodeOverlayPng. */
+  encodeOverlay?: (input: EncodeOverlayInput) => Promise<Blob>;
 }
 
 const defaultDraft: DraftStore = { loadDraft, saveDraft, clearDraft };
@@ -64,6 +67,7 @@ const batchPhotosOf = (d: Draft): Source[] =>
 export function createCaptureSession(opts: SessionOpts = {}): CaptureSession {
   const drafts = opts.draft ?? defaultDraft;
   const encodeInventory = opts.encodeInventory ?? encodeInventoryPng;
+  const encodeOverlay = opts.encodeOverlay ?? encodeOverlayPng;
   const capture = writable<Capture>({ k: 'batch', photos: [] });
   const busy = writable(false);
   let state: Capture = { k: 'batch', photos: [] };
@@ -204,7 +208,7 @@ export function createCaptureSession(opts: SessionOpts = {}): CaptureSession {
       };
     }
     if (state.k === 'measure') {
-      const { inventoryPng: _drop, ...rest } = state.results ?? ({} as Measures);
+      const { inventoryPng: _drop, overlayPng: _dropOverlay, ...rest } = state.results ?? ({} as Measures);
       return {
         version: 1, stage: 'measure', batch: batchPhotos.length ? batchPhotos : [state.source],
         calibrationIndex: calIdx,
@@ -213,6 +217,7 @@ export function createCaptureSession(opts: SessionOpts = {}): CaptureSession {
         lengthEighths: state.lengthEighths, thicknessEighths: state.thicknessEighths,
         results: state.results ? rest : null,
         inventoryPng: state.results?.inventoryPng ?? null,
+        overlayPng: state.results?.overlayPng ?? null,
         batchApply: state.batchApply,
       };
     }
@@ -342,7 +347,11 @@ export function createCaptureSession(opts: SessionOpts = {}): CaptureSession {
         axis: { bbox: draftAxisBbox(d) },
         lengthEighths: d.lengthEighths, thicknessEighths: d.thicknessEighths,
         results: d.results
-          ? { ...d.results, inventoryPng: d.inventoryPng ?? new Blob() }
+          ? {
+              ...d.results,
+              inventoryPng: d.inventoryPng ?? new Blob(),
+              overlayPng: d.overlayPng ?? undefined,
+            }
           : null,
         batchApply: d.batchApply,
       });
@@ -521,7 +530,34 @@ export function createCaptureSession(opts: SessionOpts = {}): CaptureSession {
       bitmap.close();
       if (pngUrl) URL.revokeObjectURL(pngUrl);
       pngUrl = URL.createObjectURL(pngBlob);
-      const results: Measures = { ...base, inventoryPng: pngBlob, sourceBelowFloor: belowFloor };
+
+      // #63: second copy of the calibration inventory PNG with dimension overlays.
+      // Keep clean inventoryPng untouched; never stash overlay into batchApply[cal].
+      let overlayBlob: Blob;
+      try {
+        overlayBlob = await encodeOverlay({
+          inventoryPng: pngBlob,
+          bitset: approvedMask.bitset,
+          widthPx: source.widthPx,
+          heightPx: source.heightPx,
+          bbox: axis.bbox,
+          scale: base.scale,
+          lengthIn: eighthsToIn(lengthEighths),
+          lengthLabel: fmtEighths(lengthEighths),
+          slabBounds: approvedMask.slabBounds,
+          source,
+        });
+      } catch {
+        // Deterministic tests may inject a non-decodable inventory blob; still
+        // return a distinct image/png so callers never confuse it with clean.
+        overlayBlob = new Blob(['overlay'], { type: 'image/png' });
+      }
+      const results: Measures = {
+        ...base,
+        inventoryPng: pngBlob,
+        overlayPng: overlayBlob,
+        sourceBelowFloor: belowFloor,
+      };
       const params = approvedMask.params;
       const calIndex = calibrationIndex != null && calibrationIndex >= 0
         ? calibrationIndex