LR-MERGE-PANO: a panorama of raws keeps the as-shot white balance of its frames (#1144)

A raw whose white balance is only in the vendor's multipliers (CR2, NEF, ARW...)
has no AsShotNeutral of its own. Loading a frame dropped the multipliers and
stitch copied the first frame's colour tags, so a panorama of frames that reach
the stitcher without a neutral wrote a DNG with neither AsShotNeutral nor
AsShotWhiteXY, and a viewer had no white to start from (a strong colour cast
until White Balance > Auto).

Frames now carry the multipliers (Frame::wb_multipliers, filled from the decoded
raw; load_frame_with is split so a decoded raw can be tested without a file).
stitch takes the median R/G and B/G of the multipliers of the frames that are in
the panorama (the mean of the middle two for an even count; frames without
multipliers, or with non-finite or non-positive ones, do not vote) and sets the
result's AsShotNeutral to their reciprocal, green = 1. The source's
AsShotWhiteXY is dropped with it: a reader takes it before the neutral and the
DNG spec makes the two exclusive. With no multipliers anywhere (DNG sources) the
first frame's tags are kept, as before.

In the app and the CLI a raw without colour matrices already gets a neutral from
starting_colour_tags (#965). For frames balanced alike the new neutral renders
the same (checked in the engine crate); it differs when the camera balanced the
frames differently: the panorama follows the median instead of the first frame.

Tests: the frame carries the multipliers; the consensus for odd and even counts,
with missing and unusable values; stitching raw-style frames gives the median
AsShotNeutral, ignores frames outside the panorama and leaves the colour as it
was without multipliers; the written DNG, read back with the raw reader, has the
reciprocal AsShotNeutral and the white balance the source's multipliers give.

docs/camera-preview-colour.md

@@ -194,6 +194,17 @@ as the fitted table does, and the chroma curve has no DNG tag, so it is left out
 HDR merge of three copies of the photo against the photo alone (200 px, CIE76): mean ΔE 1.07 and 0.47, 95th
 percentile 2.65 and 1.15 (26 and 23 before, with no colour tags; 1.47 and 0.66 with the table left out too).
 
+A raw whose white balance is only in its vendor multipliers (CR2, NEF, ARW…) has no `AsShotNeutral` of its own, so a
+frame carries the multipliers (`Frame::wb_multipliers`) next to its colour tags. A panorama takes its white balance
+from the frames that are in it: the median R/G and the median B/G of their multipliers (the mean of the middle two
+for an even count; frames without usable multipliers, or outside the panorama, do not vote), written as
+`AsShotNeutral`, the reciprocal, and any `AsShotWhiteXY` of the source dropped (a reader takes it first, and the
+DNG spec has the two exclude each other). So the panorama is balanced for what the camera measured for most of its
+frames, not for whichever frame came first. With no multipliers anywhere (DNG sources) the first frame's tags are kept
+as before. For frames balanced alike this is the neutral `starting_colour_tags` already wrote, so the merged DNG
+renders as before; it matters when the camera balanced the frames differently, and for a caller that loads frames
+without a colour hook (`load_frame`), whose raw frames carried no white balance at all (issue #1144).
+
 ## Nikon crop and preview colour metadata
 
 Nikon maker-note `CropArea` (0x0045) supplies the default `[left, top, width, height]` crop. The decoder validates the rectangle against the active sensor area and falls back to that area for missing or invalid values. The CFA origin is unchanged; cropping follows demosaicing.

docs/parity-checklist.md

@@ -1,6 +1,6 @@
 # Lightroom parity checklist
 
-> **Last reviewed:** 2026-10-10 · **Last updated:** 2026-10-11 · **Change:** minor (LR-MERGE-HDR: merges of raws without colour matrices keep their colour (#965); rapid slider entry; LR-PROF-CAMERACOLOR: tone curve from black-and-white camera JPEGs; LR-VIEW-ZOOM, LR-VIEW-COMPARE: Compare and Reference zoom windows; LR-IMP-FORMATS, LR-IMP-CAMERA-COVERAGE: EOS R5 Mark II CR3 decodes; renamed from `parity.md`; Top gaps moved to gaps.md, shortcut notes to ui-parity.md; filesystem history retry/Undo/Redo evidence; renamed from `parity.md`; Top gaps moved to gaps.md, shortcut notes to ui-parity.md) · **Target:** Adobe Lightroom Classic 15.5 / Lightroom 9.6
+> **Last reviewed:** 2026-10-10 · **Last updated:** 2026-10-13 · **Change:** minor (LR-MERGE-PANO: a panorama of raws takes its as-shot white balance from the median of its frames (#1144); LR-MERGE-HDR: merges of raws without colour matrices keep their colour (#965); rapid slider entry; LR-PROF-CAMERACOLOR: tone curve from black-and-white camera JPEGs; LR-VIEW-ZOOM, LR-VIEW-COMPARE: Compare and Reference zoom windows; LR-IMP-FORMATS, LR-IMP-CAMERA-COVERAGE: EOS R5 Mark II CR3 decodes; renamed from `parity.md`; Top gaps moved to gaps.md, shortcut notes to ui-parity.md; filesystem history retry/Undo/Redo evidence; renamed from `parity.md`; Top gaps moved to gaps.md, shortcut notes to ui-parity.md) · **Target:** Adobe Lightroom Classic 15.5 / Lightroom 9.6
 
 The measured, row-by-row checklist: one row per Lightroom feature, menu item and shortcut, what we
 have and what is missing. It answers *does the feature exist?* (feature breadth). Whether it is good
@@ -348,7 +348,7 @@ differences and missing bindings are in [ui-parity.md](ui-parity.md#shortcuts).
 | Id | Feature | Tier | Status | Evidence | Missing / notes |
 |---|---|---|---|---|---|
 | LR-MERGE-HDR | HDR merge | P2 | ✅ | `cmd:merge.hdr`, `cmd:dialog.mergeHdr` | auto align, deghost None–High + overlay, auto settings, float DNG, Create Stack; JPEG brackets treated as linear; raws without colour matrices of their own (CR2, NEF, ARW…) keep the colour they open with, written as DNG tags (`starting_colour_tags`; the look's chroma curve has no tag; panoramas too), issue #965 |
-| LR-MERGE-PANO | Panorama | P2 | ✅ | `cmd:merge.panorama`, `cmd:dialog.mergePanorama` | spherical/cylindrical/perspective + auto, boundary warp, auto crop, fill edges (diffusion), DNG; no lens model / 360° wrap |
+| LR-MERGE-PANO | Panorama | P2 | ✅ | `cmd:merge.panorama`, `cmd:dialog.mergePanorama` | spherical/cylindrical/perspective + auto, boundary warp, auto crop, fill edges (diffusion), DNG whose `AsShotNeutral` is the median of its frames' as-shot white balance (vendor multipliers for CR2, NEF, ARW…), issue #1144; no lens model / 360° wrap |
 | LR-MERGE-HDRPANO | HDR panorama | P2 | ✅ | `cmd:merge.hdrPanorama`, `cmd:dialog.mergeHdrPanorama` | brackets grouped by EXIF |
 | LR-MERGE-HEADLESS | Merge with last settings | P2 | ✅ | `cmd:merge.hdrLast`, `cmd:merge.panoramaLast`, `cmd:merge.hdrPanoramaLast` | no dialog; the options of the last merge of that kind (defaults the first time) |
 
@@ -789,6 +789,7 @@ Latin stays upright. Advanced Japanese composition remains open. Regression cove
 
 | Date | Change | Summary |
 |---|---|---|
+| 2026-10-13 | minor | LR-MERGE-PANO: a panorama's DNG takes its `AsShotNeutral` from the median of its frames' vendor white-balance multipliers (CR2, NEF, ARW…) instead of the first frame's tags, and frames now carry the multipliers (issue #1144); status unchanged (✅) |
 | 2026-10-11 | minor | LR-MERGE-HDR: HDR and panorama merges of raws without colour matrices (CR2, NEF, ARW…) write the colour they open with as DNG tags (issue #965); status unchanged |
 | 2026-10-12 | minor | LR-META-XMP: Read Metadata from File no longer auto-writes the sidecar it has just read, which added `lc:settings` and hid another app's later `crs:` edits (#934); status unchanged (✅) |
 | 2026-10-12 | patch | LR-IMP-CAMERA-COVERAGE: CR2 white level from the active area, not the saturated masked rows above it (issue #1272); status unchanged (🟡) |