fix(fel): require per-step weights non-negative over the counter range

[namasikanam]

Summary

The failure-event lemma tactic (fel) is unsound: it enforces neither that the counter
increments by exactly 1 per bad event, nor that each per-step weight is non-negative at the
indices it skips. A counter that jumps 0 -> 2 with weights (1, -1) sums to 0, so fel
"proves" Pr[bad] <= 0 while the true probability is 1 — hence false.

Root cause

t_failure_event_r sums the per-step weights only over the indices the counter actually
visits, and never requires 0%r <= ash i on the whole range 0 <= i < q. Negative weights
at skipped indices are therefore uncounted.

Fix (src/phl/ecPhlFel.ml)

Add the side-condition forall i, 0 <= i < q => 0%r <= ash i (emitted as the last goal).
Existing library/examples uses of fel are updated to discharge this trivially-true
obligation (all standard weights are already non-negative).

Test

tests/ko/fel-counter-jump.ec (must-fail): the 0 -> 2 counter jump with a negative
weight is now rejected.