docs(spec): make the count zero-rule context-dependent per consumer (0083)

The count description claimed every count must be "positive integral",
which is wrong: zero is context-dependent. Verified at HEAD — an array
dimension (`[0]s64`) and a generic value-param count (`Box(0)`, $N:u32)
both accept zero as a length-0 instantiation, while a `Vector` lane
count stays strictly positive (`Vector(0,f32)` rejected). Negatives are
rejected for array dims and unsigned value-params, but a signed
value-param accepts a negative; only the integral requirement (folds
4.0, rejects 4.5) is common to all three.

Split the count paragraph into per-consumer bullets stating the exact
range each accepts. Range-bound paragraph unchanged. Pin the zero
contrast with examples 0147 (array-dim + value-param zero accepted) and
1505 (Vector zero-lane rejected). No compiler-code change.

examples/expected/0147-types-zero-count-context.exit

@@ -0,0 +1 @@
+0

examples/expected/0147-types-zero-count-context.stdout

@@ -0,0 +1,2 @@
+array_dim=0
+value_param=0

examples/expected/1505-vectors-zero-lane-rejected.exit

@@ -0,0 +1 @@
+1

examples/expected/1505-vectors-zero-lane-rejected.stderr

@@ -0,0 +1,5 @@
+error: Vector lane count must be a positive compile-time integer constant
+  --> examples/1505-vectors-zero-lane-rejected.sx:13:16
+   |
+13 |     v : Vector(0, f32) = ---;
+   |                ^