Specify panorama merging: §3.11, D18, S15, and the design in panorama.md

A merge writes a new source file beside its sources (D18) rather than a
multi-source Version, which answers the schema question §7 had been holding
open for panorama, HDR merge and focus stacking together. The panorama is
undeferred as FR-MRG-1 … 11; the other two stay in §7 with their data model
decided.

FR-MRG-10 and 11 fix where the work runs — every per-pixel stage on the GPU,
the composite never held as one texture — because the output exceeds
max_texture_dimension_2d before it exceeds memory. panorama.md carries the
stage table, the chunked output driver, the model licences and the porting
sources. S15 gates all of it.

Coverage falls from 83.0% to 77.2%: thirteen requirements entered with no
code, and outstanding.md §11 says so.

docs/outstanding.md

@@ -455,7 +455,20 @@ whether something *should* be built — which is the opposite of the order §9 a
 
 ---
 
-## 11. D12, which governed all of the above
+## 11. Merging — specified 2026-09-19, nothing built
+
+§3.11 was written on 2026-09-19 under D18, undeferring the panorama from §7 and leaving HDR merge
+and focus stacking there with their data model decided. Eleven `FR-MRG` clauses and two `NFR-MRG`
+figures entered the register at once with no code behind any of them, which is why the coverage
+figure fell from 83.0% to 77.2% on the same day — a specification, not a regression.
+
+[panorama.md](panorama.md) is the design, and its §10 is the order of work. Nothing starts before
+**S15**: whether rawler reads back a linear DNG the application writes, whether XFeat loads under
+tract at a fixed shape, whether the working-space texture can be tapped where FR-MRG-2 needs it,
+and what a chunked blend of a 100 MP composite costs on the tablet. The first two are a day each
+and either can change the design, which is the reason they come first.
+
+## 12. D12, which governed all of the above
 
 > **Decided 2026-09-19, by events.** The scope stands as calibrated, v1 has no date, and `(post-v1)`
 > in §7 is the one way a clause leaves the count — used for the plugin API and nothing else. The