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/finance/phase1-construction-rfq.md

@@ -30,7 +30,7 @@ class, not a premium — it is what the presses and pallet racking need, and
 stepping up from a light-duty slab adds only on the order of 100–200 THB/m².
 The real cost variable is the ground underneath (see soil, below). A later
 value-engineering option is to zone the slab — 30 kN/m² under press hall and
-warehouse only, lighter under offices — but for rough pricing the whole
+warehouse only, lighter under offices, but for rough pricing the whole
 floor is specified at 30 so the number is conservative.
 
 ### 1a. Panel core: where PIR is not enough
@@ -109,7 +109,7 @@ Simple industrial enclosures beside the main building.
 | Boiler hall | 66 m² | 6.0 m |
 | Biomass fuel store | 42 m² | 5.0 m |
 | Chiller room | 60 m² | 5.0 m |
-| Cooling tower pad (concrete pad only) | 10 m² | — |
+| Cooling tower pad (concrete pad only) | 10 m² | - |
 
 The 14 m flue stack comes with the boiler — excluded here. The boiler hall
 envelope should be non-combustible (mineral-wool panels or masonry), not
@@ -165,7 +165,7 @@ transformer (PEA), and land works beyond normal grading.
   soft-to-medium Lamphun valley soil and to **state the foundation
   assumption they price** (footings vs driven piles; a piled slab can add
   several hundred to 1,000+ THB/m²). The soil, not the slab spec, is the
-  main price risk in the whole package — and it now applies to 4,387 m² of
+  main price risk in the whole package, and it now applies to 4,387 m² of
   pavement and a 1,711 m³ fill platform as well as the building.
 - **Tank proximity to the building**: the fire water tank sits about 4.5 m
   from the main building. Full, it is ~250 t on a 9 m ring foundation. On