lang F1: range-based for + inline-for unroll over packs

Add range loop syntax:
- runtime  for start..end (i) { }   counting loop, cursor optional, end exclusive
- comptime inline for start..end (i) { }   comptime-unrolled body

The inline form binds the cursor as an int_val comptime constant per
iteration, so xs[i] over a heterogeneous pack substitutes the concrete
per-position element -- the canonical's pack-iteration vehicle
(inline for 0..sources.len (i) { sources[i].addListener(...) }).

- AST: ForExpr.range_end, ForExpr.is_inline
- parser: parseForExpr range vs collection form; suppress_call flag so
  N (i) is not read as a call N(i) while parsing a range bound
- lower: lowerRuntimeRangeFor / lowerInlineRangeFor; evalComptimeInt;
  comptimeIndexOf extends pack-index resolution beyond int literals

Revises spec's inline for i in 0..N to the no-in, range-first, paren-cursor
form. Regression: examples/200-for-range.sx.

tests/expected/200-for-range.exit

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

tests/expected/200-for-range.txt

@@ -0,0 +1,10 @@
+i=0
+i=1
+i=2
+n=5
+j=2
+j=3
+j=4
+[0]=A
+[1]=B
+[2]=A