support range-for: for i in lo..hi

[kacy]

summary

counted iteration previously meant a manual mut i := 0; while i < n loop or
materializing a list with range(). this adds native range syntax that lowers
to a counted loop with no allocation:

for i in 0..n:      # exclusive, i in [0, n)
for i in 0..=n:     # inclusive, i in [0, n]

bounds may be any Int expression (for i in 1..n + 1:), the optional
value, index binding works (the index is the 0-based position), and break
/ continue behave as in any other loop.

examples/matrix_math.pith is reworked from while counters to range-for as
a worked example (its output snapshot is unchanged).

how

• lexer: . now lookaheads to emit dotdot (..) and dotdoteq (..=)
• parser: parse_for_stmt recognizes the range operator in the iterable
  position and builds a range node. range stays scoped to for-iterables for
  now, so the rest of the expression grammar is untouched.
• checker: a range iterable requires Int bounds and binds the loop
  variable to Int (E217 otherwise)
• ir emitter: ir_emit_for_stmt detects the range node and emits a counted
  loop directly — init the counter to lo, compare lt/lte against hi,
  bind the loop variable, step by one — reusing the existing break/continue
  label scaffolding.

what was tested

• new regression tests/cases/test_range_for.pith covering exclusive,
  inclusive, empty (3..3), expression bounds, break/continue, and the
  value, index binding
• new invalid-checker case invalid_range_bounds asserting E217 on non-Int
  bounds
• full suite: regressions (93), invalid checker (13), invalid parse (3),
  examples via native and self-hosted compilers (83 each), and bootstrap fixed
  point — all 0 failures

notes

range is intentionally limited to the for-iterable position in this change.
generalizing it to a first-class lazy value belongs with the iterator-protocol
work, which would also let .map().filter() fuse without intermediate lists.