docs(replicate): seq is an interval index, not a frame counter

[brayniac]

The doc said Frame::Rows.seq "counts from zero per source per connection". The
code needs less than that:

let gap = match st.last_seq {
    Some(last) => seq > last.saturating_add(1),
    None => false,
};

last_seq starts None, so a first value is never compared, and the only check
is the step between consecutive frames. Nothing anywhere depends on the starting
value.

Why the over-promise had a cost

A consumer's publisher sends the subscription's interval index — the
observation's timestamp divided by the interval the subscriber asked for. It
satisfies the gap check identically and carries strictly more information, and
it read as non-conforming against the letter of the doc.

They're right, and the doc was mandating the weaker option. A frame counter
increments by one whether or not an interval was skipped, so a skipped interval
arrives as a contiguous sequence with an undetectable hole in the middle.
Rule 6 says a gap means a lost reading; with a counter it can only mean a lost
frame in transit. An interval index makes rule 6 mean what it says.

One thing added beyond the report

Derive the index from the row timestamp, not the wall clock. FORMAT.md §5
anchors ts to a monotonic clock precisely so it survives a wall-clock step; an
index taken from the wall clock inherits the step and can go backwards.

That matters more than it looks, because a backwards value is not reported
today. The check is seq > last + 1, so a repeat or a regression reads as "no
gap", is applied silently, and regresses last_seq. With a frame counter from a
well-behaved publisher that cannot arise; once the contract admits a
clock-derived index, it can. The docs now name that gap rather than leaving it
implicit.

Whether the subscriber should detect a non-increasing seq is a separate
question and a behaviour change, so it is not in this PR.

Scope

Docs only — frame.rs, mod.rs rule 6, WIRE.md §4.3, CHANGELOG. No code
change, and a publisher that counts from zero still conforms.

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