feat: выбор граней окон для солнечных лучей (#577)

Issue: #577
User-Visible: yes

docs/SUN.md

@@ -204,17 +204,24 @@ when BOTH hold:
   perpendicular depth (`len · cos`, see «Dissolving») would be thinner
   than the wall it came through.
 
-The wedge is a PARALLELOGRAM: the window's full **room-side span**, from
-one inner corner of the opening to the other, extruded by the same
+The wedge is a PARALLELOGRAM. The global `settings.sun_ray_origin` enum selects
+its full source span: `inner` starts at the two room-side corners (the default
+and the exact pre-#577 behaviour), while `outer` starts at the two exterior
+corners. Missing or unknown values resolve to `inner`; this setting has no
+per-space override. The selected span is extruded by the same
 length along the direction AWAY from the sun (light falls inward), so
 its far edge is parallel to the wall, clipped by the receiving room's
 **inner contour** when wall thickness is set (`inset` of the polygon by
 half the wall thickness — see `docs/WALL-THICKNESS.md`); otherwise by
-the room polygon (`polyclip` intersection). With wall depth `d`, the
-source span is translated from the wall centreline by `d/2` along the
-inward normal. Thus both crisp side edges begin exactly at the two inner
-corners of the opening at every incidence angle (`d = 0` keeps the
-previous centreline/full-span geometry). Its length
+the room polygon (`polyclip` intersection). In `outer`, the clip also includes
+only the physical rectangular window tunnel between the exterior span and the
+clean-floor contour; the surrounding wall body and outside facade remain
+occluders. With wall depth `d`, `inner` translates the source from the
+centreline by `+d/2` along the inward normal and `outer` by `-d/2`. Thus both
+crisp side edges begin exactly at the selected two corners at every incidence
+angle (`d = 0` makes both modes identical). The nominal length and gradient
+start at that selected span rather than adding the wall depth to the old reach.
+Its length
 is `k(elevation)` in window lengths: ~1.75 at sunrise/sunset tapering to ~0.56 at the zenith
 (`0.56 + 1.19·(1 − elevation/90)^1.6` — the v1.56 curve
 `0.8 + 1.7·(1 − elevation/90)^1.6` times `RAY_LENGTH_K` = 0.7, owner
@@ -245,10 +252,10 @@ Two rounds with the owner on the same day:
    wedge, which feathered the SIDES too. Wrong: a shaft of sunlight
    through a window has crisp sides. Only its reach fades.
 
-So the falloff is one-dimensional: **along the ray, from the room-side opening
+So the falloff is one-dimensional: **along the ray, from the selected opening
 inward, and nothing else.** Three invariants have to hold at once:
 
-1. the whole room-side opening is at peak alpha — light does not start out
+1. the whole selected opening span is at peak alpha — light does not start out
    half-dark at one end of the window;
 2. every ray fades over the same distance, its own `len`;
 3. the wedge's far edge lies exactly on an iso-alpha line, so the shaft
@@ -257,21 +264,21 @@ inward, and nothing else.** Three invariants have to hold at once:
 
 **The axis of the fade is the wall's INWARD NORMAL, not the ray.**
 The light is a bundle of PARALLEL rays, so the distance a point has
-travelled from the room-side source span is `depth / cos`, where `depth`
+travelled from the selected source span is `depth / cos`, where `depth`
 is its perpendicular distance from that span and `cos = dir·normal` is fixed
 for the whole wedge. That is an affine function of the point, and its
 level sets are straight lines PARALLEL TO THE WALL. A linear gradient
 whose axis is the normal therefore describes the travelled distance
 exactly:
 
-- `x1,y1` = the middle of the room-side window span (any point of that span —
+- `x1,y1` = the middle of the selected window span (any point of that span —
   they all have depth 0);
 - `x2,y2` = that point plus `normal · len · cos` — `SunRay.depth`, the
   perpendicular depth a ray reaches after running the full `len`;
 - a point `source + dir·u` lands on offset `u / len`, whichever ray it
   rode in on.
 
-Hence: the inner opening span is all at offset 0 (invariant 1), the alpha at any
+Hence: the selected opening span is all at offset 0 (invariant 1), the alpha at any
 point is a function of how far its own ray has run (invariant 2), and
 the parallelogram's far edge — parallel to the wall — IS the gradient's
 last iso-alpha line (invariant 3). The «30 % shorter» reach is then a
@@ -319,7 +326,7 @@ of its cheap half, verbatim: «тонкая (1px) чёрная граница п
 The contract:
 
 - **Two side edges only.** The rim runs along the two edges that leave
-  the inner opening corners and travel inward with the ray — `a → a+dir·len`
+  the selected opening corners and travel inward with the ray — `a → a+dir·len`
   and `b → b+dir·len`. Never the source edge `a-b` (that is the source,
   not a boundary) and never the far edge (there is nothing left to
   outline there — the fill is already at zero, see below).
@@ -331,7 +338,7 @@ The contract:
   `x1,y1,x2,y2`** (the wall's inward normal, `depth` long) and **the same
   normalised curve** — `rimStops()` returns `rayStops()` by identity, not
   by copy, so the two can never drift apart. Only the colour and the peak
-  differ: `RIM_MAX_ALPHA` = 0.42 at the inner opening, tuned on the demo rig
+  differ: `RIM_MAX_ALPHA` = 0.42 at the selected opening, tuned on the demo rig
   against both extremes (below ~0.3 the line vanishes on paper at kiosk
   scale, above ~0.5 it reads as an ink contour over the dark glow
   canvas). Zero from `RAY_FADE_END` = 85 % on, like the fill.