Repair the references that point at sections that moved

Eight citations named §5.1, §5.2 and a §5 selector language that
requirements.md's §5 has not contained since it became a pointer at
architecture.md; two named §9 for the golden images and the benchmark
suite, which are §8; and the three pointers into architecture.md were
each one section off. All now name the section that holds the thing.

architecture.md §12's subsections are numbered 6.1–6.13, colliding with
its real §6. That numbering is what every ARCH §6.n citation in the tree
uses, so it stays, and a note at the head of §12 says so instead of
leaving the next reader to work it out.

FR-DEV-3f's open question about persisting the film stock was answered
in sidecar.rs; the clause now says so.

docs/architecture.md

@@ -1099,6 +1099,13 @@ architecture; everything else assumes they pass.
 
 Decisions treated as fixed because reversing them is expensive. Each is evidence-backed.
 
+> **On the numbering.** The subsections below are numbered **6.1–6.13**, not 12.x. They were §6
+> when this document was first written, and every `ARCH §6.n` citation in `requirements.md`, in
+> `docs/*.md` and in source comments names them by those numbers. Renumbering would break several
+> hundred citations for no gain, so the numbers stay: `ARCH §6.1` means the first constraint here,
+> and never §6.1 of the data architecture above, which is cited as *§6 Data architecture* or by
+> its title.
+
 ### 6.1 GPU results never round-trip through the CPU
 
 darktable identifies this as their single biggest bottleneck: OpenCL output returns to the GTK