Leave the profile's look table off by default

Every raw rendered through a camera profile — the library's DNGs with an
embedded profile, and CR2s given one — now renders differently: more
colourful in near-neutral tones. The profile's look table is no longer
applied unless its slider is raised; PROFILE_LOOK names the strength the
profile states.

Against the photographer's earlier exports with no look applied, the default
rendering scores the same with the look table at 100, 50 or 0 (held-out MSE
140, 140, 143), and is 9 % more colourful at 0: the table lowers the
saturation of near-neutral tones, which is exactly where the default
rendering was short of those exports. The user chose more colour.

docs/dev/requirements.md

@@ -2801,7 +2801,7 @@ curve a power of 1.5/1.4 about grey (`REFERENCE_CONTRAST` is where the curve is
 brightness needs nothing: baseline exposure and the curve together land where the earlier exports do. The
 profile's look strength, vibrance and saturation bought nothing measurable on those exports. The
 user chose to change every photograph rather than keep edited ones on the old rendering. The
-fitting tools live outside the repository (`darkroom-lrfit`).
+fitting tools live outside the repository (`darkroom-lrfit`). *Amended 2026-10-04:* the look strength now defaults to 0. It scored the same at 100, 50 and 0 (held-out MSE 140, 140, 143) and the rendering is 9 % more colourful without it — the table desaturates near-neutral tones, where the default was short of those exports; the user chose more colour.
 
 *Amended earlier the same day:* the default was **not** decided. The measurement below was against
 Lightroom previews of photographs carrying the user's Lightroom edits — a house look of HSL