Remove em dashes where the replacement is unambiguous

The project has a standing style rule against em dashes that was missed
when these docs were written. This removes the 42 occurrences where the
substitution is mechanical and cannot change meaning:

- table cells using an em dash as a placeholder, now a hyphen
- em dashes immediately followed by a conjunction or subordinator
  (but, and, or, so, though, while, which, where, because, since),
  where a comma is simply correct

115 remain. Those join independent clauses, where a comma would create a
splice and a colon often reads wrong, so each needs a judgement call
rather than a pattern. A blanket regex over them was attempted and
reverted: it produced double colons and broken sentences.

docs/scale.md

@@ -74,10 +74,10 @@ fuel delivery both push toward:
 | Biomass fuel store | 42 m² | 5.0 m |
 | Chiller room | 60 m² | 5.0 m |
 | Cooling tower (outdoor pad) | 10 m² | 3.5 m |
-| Flue stack | — | 14 m |
+| Flue stack | - | 14 m |
 
 Boiler plus chiller at 126 m² lands inside the 100 to 150 m² the zone table
-allows — but moving them out of the main shed frees that floor for
+allows, but moving them out of the main shed frees that floor for
 production, so either the main building shrinks by ~126 m² or the program
 grows into it. Decide which before the building is priced.
 
@@ -110,7 +110,7 @@ production floor — not a gap in it.
   loading off the phase 1 circulation road.
 - **Roof form: two-bay sawtooth, shaded rows deliberately left empty.**
   A single 38 m monopitch would be the best solar answer — 28 panel rows,
-  none shaded, ~17% more yield than what is drawn — but it needs a **13.7 m**
+  none shaded, ~17% more yield than what is drawn, but it needs a **13.7 m**
   high wall. The sawtooth holds the high side to **10.35 m**, at a cost: each
   bay falls toward ESE onto the next bay's 3.35 m riser, so the bottom rows
   sit in shadow (the lowest is blocked below a 53° sun profile angle). Those