fix(panel): pivotalness null where ownership is unknown, not "not pivotal"

[edwinhu]

The four pivotalness flags were abs(forpct-50) <= coalesce(ior,0).

That coalesce turns "we have no denominator for this firm" into "this firm's owners hold nothing" at the moment the flag is set — so a row with no measurable ownership is recorded as known not pivotal, indistinguishable from one measured at 0.4% and genuinely not pivotal.

It re-conflates downstream exactly what the upstream null protects. #97 stopped leg 2 writing ior = 0.0 for an unknown denominator; this put the zero back one join later. 42.85% of leg-2 rows have no CRSP denominator, and the ISS-side tso has its own gaps — a large minority of the panel, not an edge.

inst_pivotal_net four lines below already did it correctly, and its comment states the entire argument:

null where lending is unknown, so it cannot quietly fall back to the gross measure

Same reasoning, four columns earlier. The right remedy was already written in the same paragraph as the wrong one.

Digest impact: inst_pivotal / mf_pivotal / passive_pivotal / index_pivotal go from {0,1} to {0,1,.}. B moves on those four columns by design; nothing else should.