lang: range bound markers — '=' inclusive / '<' exclusive on either side of '..'

Each side of '..' takes an optional bound marker, defaulting to
start-inclusive, end-exclusive (a..b == a=..<b; a..=b stays the short
end-inclusive spelling):

    for 0<..<N (i) { }   // 1 .. N-1   (both exclusive)
    for 0=..=N (i) { }   // 0 .. N     (both inclusive)
    for 0<..=N (i) { }   // 1 .. N
    for 0..<N  (i) { }   // 0 .. N-1   (explicit default)
    for xs, 2<.. (x, i)  // open range, exclusive start: i = 3, 4, ...

The nine lexemes are single tokens (maximal munch on '<'/'='/'..'), so
expression parsing never sees the leading marker as a comparison; '<',
'<<', '<=', '==', '=>' lex unchanged. An explicit end marker makes the
end expression mandatory; open forms are a.. / a<.. / a=... Works in
runtime, multi-iterable, and inline-for headers.

Regression: examples/0051-basic-for-range-bounds.sx (full matrix, open
start-marked ranges, comptime unroll, runtime bounds, lexer
non-regression); 1152's pinned message generalized.

src/ast.zig

@@ -646,13 +646,19 @@ pub const WhileExpr = struct {
 pub const ForIterable = struct {
     /// Collection expression, or the range START for the range forms.
     expr: *Node,
-    /// `a..b` / `a..=b` end. Null for a plain collection AND for the
-    /// open-ended range `a..` (distinguished by `is_range`).
+    /// Range end. Null for a plain collection AND for the open-ended range
+    /// `a..` (distinguished by `is_range`).
     range_end: ?*Node = null,
-    /// True for any range form (`a..`, `a..b`, `a..=b`).
+    /// True for any range form. Each side of `..` takes an optional bound
+    /// marker — `=` inclusive, `<` exclusive — with defaults start-inclusive,
+    /// end-exclusive: `a..b` ≡ `a=..<b`; `a<..<b` is 1-past-start to
+    /// end-1; `a=..=b` includes both ends; `a..=b` keeps the short
+    /// end-inclusive spelling.
     is_range: bool = false,
-    /// `a..=b` — end is inclusive.
-    inclusive: bool = false,
+    /// `<..` family — start is exclusive (cursor begins at start+1).
+    start_exclusive: bool = false,
+    /// `..=` family — end is inclusive.
+    end_inclusive: bool = false,
 };
 
 /// One capture of a `for` header: `(x)`, `(*x)`, `(x, y, ...)`.