Implement loop (infinite) and do...while/until loops

Completes the 'loop, next, break, do' TODO entry — break and next
were landed in the previous commit; this adds the remaining two forms:

- loop { body } — infinite loop; the only exit is break. Simpler than
  while/until since there is no condition expression to evaluate.

- do { body } while expr; / do { body } until expr; — post-condition
  loop: body always executes at least once, condition is checked at the
  bottom of each iteration. Condition tokens are collected until ';'
  (mirrors _expression_get but stops at semicolon instead of '{'), then
  replayed each iteration using the same temp-stack/temp-iterator
  technique as while/until. Both break and next are supported.

Token changes: TT_LOOP and TT_DO added to the keyword enum in token.h,
registered in get_tt() and tt_get_name() in token.c.

Add examples/loop_do.fy; expected output: 5 / 12 / 11 / 5.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

1 files changed, 32 insertions, 0 deletions

diff --git a/examples/loop_do.fy b/examples/loop_do.fy
new file mode 100644
index 0000000..60cfa5a
--- /dev/null
+++ b/examples/loop_do.fy
@@ -0,0 +1,32 @@
+# loop — infinite loop, break exits after 5 iterations
+my i = 0;
+loop {
+    i = i + 1;
+    if i == 5 { break; }
+}
+say i;          # expected: 5
+
+# loop with next — skips j==3, so sum = 1+2+4+5 = 12
+my sum = 0;
+my j = 0;
+loop {
+    j = j + 1;
+    if j > 5 { break; }
+    if j == 3 { next; }
+    sum = sum + j;
+}
+say sum;        # expected: 12
+
+# do...while — body runs once even though k >= 10 already
+my k = 10;
+do {
+    k = k + 1;
+} while k < 10;
+say k;          # expected: 11
+
+# do...until — stops when m reaches 5
+my m = 0;
+do {
+    m = m + 1;
+} until m == 5;
+say m;          # expected: 5