feat: warn where a pair connects but falls short of a feature

The matrix marked every accepted pair with a plain tick, which reads as
"everything works". It does not. ProtocolVersion bumps PATCH for sub-channels a
peer can safely ignore, so Paper 1.3.0 (protocol 1.0.1) against Velocity 1.1.0
(1.0.0) completes the handshake and then silently drops cross-server direct
messages — the very feature that PATCH was bumped for. An operator reading the
tick had no way to learn that, and the pages around it repeated the claim.

The handshake is settled by MAJOR and MINOR alone, so any difference left once a
pair is accepted is a feature the newer end offers and the older will never
answer. Those pairs now carry a warning that says which end lags. Naming the
feature would take a protocol-version-to-feature table on the website, which
would drift from the protocol it describes, so the warning stays general and
leaves the reader one hop from the compatibility page.

The handshake verdict keeps the three checks that mirror Velocity's gate, with
the new one layered after them, so the mirror stays honest. isCompatible now
holds for a degraded pair, which does connect.

Cells are built once per pair in a computed instead of recomputing on each of the
template's reads, which a third state would otherwise have multiplied.

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

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