Fail sidechain proposals that can no longer reach the activation threshold

[1440000bytes]

BIP300 lists four conditions under which a sidechain proposal fails. The code only implemented the two max-age cutoffs; it omitted the two MAX_FAILS early-failure conditions.

bip300301_enforcer/lib/validator/task/mod.rs

Lines 276 to 277 in cbc1f80

	// FIXME: Do we need to check that the vote_count is below the threshold, or is it
	// enough to check that the max age was exceeded?

A proposal should fail as soon as it has accumulated enough non-ack blocks that it can no longer reach the activation threshold in its remaining window (max_fails = max_age - threshold; 201 for unused, 13150 for used). Otherwise a doomed proposal lingers until its max age and because handle_m1 ignores a re-proposal of any still-existing proposal, two enforcers can diverge on whether a sidechain activates during that window.

Fix

Fail a proposal when age > max_fails && (age - vote_count) >= max_fails, derived from the existing thresholds.