guidance(#356): drop the imperative-shaped clause; point at why h3_cell_area exists

"Drop it and every cell counts equally" opens with an imperative verb. A small
model reading the guide as instructions can take that as the instruction, which
is the same hazard AGENTS.md:137 records for antipattern SQL blocks — structure
reproduces regardless of the surrounding framing.

Replaced with the salient fact: cells at one resolution differ in area, which is
what h3_cell_area exists to measure. If area followed from resolution alone the
function would be unnecessary, so the guide now argues from something the model
can already see rather than from a rule it has to accept.

Author: cboettig

h3-guide.md

@@ -292,7 +292,7 @@ GROUP BY class;
 
 `get_schema` names the fractions asset and the no-data code. Per cell `SUM(frac) <= 1`; the shortfall is outside-raster/quantization, so do not treat the layer as covering 100% of a cell.
 
-**Overlaying two partial-coverage layers — multiply the coverage fractions, then weight by cell area; never count a cell as all-or-nothing.** *(Skip unless you are intersecting a fractional-coverage layer with another layer that only partly covers its cells — e.g. "what percent of each habitat class is conserved".)* Treating a cell as fully inside the other layer whenever it matches over-counts every partly-covered cell. Weight by the coverage fraction on **both** sides, and carry `h3_cell_area` through numerator and denominator. It looks like it divides out; it does not — cell area varies from cell to cell, so it will not come out of either sum. Drop it and every cell counts equally, over-counting wherever cells run smaller.
+**Overlaying two partial-coverage layers — multiply the coverage fractions, then weight by cell area; never count a cell as all-or-nothing.** *(Skip unless you are intersecting a fractional-coverage layer with another layer that only partly covers its cells — e.g. "what percent of each habitat class is conserved".)* Treating a cell as fully inside the other layer whenever it matches over-counts every partly-covered cell. Weight by the coverage fraction on **both** sides, and carry `h3_cell_area` through numerator and denominator. It looks like it divides out; it does not — cells at the same resolution differ in area, which is what `h3_cell_area` exists to measure.
 
 The other layer's per-cell weight comes from a companion **per-cell weights asset** when one exists — `get_schema` names it (e.g. ca30x30 conserved-areas publishes `…-hex-weights` at res 10, with `w1`–`w4` giving the share of each cell in GAP status 1–4). Otherwise reduce the layer's features to one weight per cell first: `MAX` over the features on the cell of the per-feature coverage share its STAC documents.