Implement break and next for while/until loops

TT_BREAK and TT_NEXT were already tokenised; this wires them up:

- _program(): guard statement loop with ct == CONTROL_NONE so a break/next
  flag set inside a loop body stops block execution and propagates upward
- _control(): add TT_BREAK and TT_NEXT cases that set CONTROL_BREAK /
  CONTROL_NEXT on p_interpret->ct and return immediately
- while/until loop: replace the commented-out switch with active if/else
  logic that clears the flag and either stops iteration (break) or lets
  the loop re-evaluate its condition (next)

Add examples/break_next.fy to exercise both keywords in while and until
loops; expected output: 5 / 12 / 7.

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

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