ANSWER: is 3 a position, or a convention we agreed on?

Both, at different layers, and the split is the same as everywhere else. The
NUMERAL is convention -- int_to_str was already form 1, because no position
determines that twelve is written 1 then 2 in base ten. The NUMBER is a
position: three things are three things regardless of notation.

But the sharper answer follows from `love = 0`. A bare `3` is a MAGNITUDE WITH
NO AXIS. It is not a position until something gives it a direction, which is
exactly why 3.days needs a calendar and why time_add(t, n, "min") had to carry
its axis as a string.

PREDICTIONS AND RESULTS
  P1 numeral = convention, number = position                TRUE
  P2 a bare literal is dimensionless until context types it TRUE
  P3 there is a measurable place where El guesses           TRUE
  P4 Instant + Int is not caught though Duration + Int is   TRUE
  P5 the rule catches it                                    TRUE
  P6 nothing legitimate in the tree relies on it            TRUE

P3/P4 IS THE DEFECT, and it was found by reasoning from the philosophy and then
measured. Duration + Int was refused -- "an Int carries no unit" -- while

    let t: Instant = now()
    let u: Instant = t + 3

compiled to raw (t + 3) and reported CLEAN. Adding a dimensionless number to a
point is worse than adding it to a displacement: it silently moves the instant
by an unspecified amount. 3 of what? Whatever the representation happens to be,
which is the leak itself. The asymmetry had no justification; the rule was
simply never written.

P6 MATTERED. Two calendar tests looked like Instant + Int:

    let later: Instant = i + 1.hour
    let later: Instant = base + 15.hours

They are not. `1.hour` lexes to a Duration -- el_duration_from_nanos(1LL *
3600000000000LL) -- and both stay clean. That is the whole answer demonstrated
in one line: t + 3 is refused because 3 has no axis; t + 1.hour is accepted
because .hour supplies one.

104/104 native + 2 new, integration green, fixpoint ok.

lang/tools/check/temporal.rel

@@ -11,6 +11,7 @@
 #   <kind> means <message>
 
 instant_plus_instant   means Instant + Instant is not allowed — a point plus a point is not a point. Subtract them for a Duration, or add a Duration.
+instant_plus_int       means Instant + Int is not allowed — a bare literal is a magnitude with no axis. 3 of what? Adding it to a point moves the instant by an unspecified amount. Use a Duration.
 duration_plus_int      means Duration + Int is not allowed — an Int carries no unit. Use duration_seconds(n) or N.seconds.
 duration_minus_int     means Duration - Int is not allowed — an Int carries no unit.
 instant_cmp_duration   means Instant < Duration is not allowed — a point and a displacement are not on the same scale.