In this commit, we land exp-021's instrument: two flag-gated changes
to lnd's own payment stack, byte-identical to stock with both flags
off (proven against a pre-change binary across four tiers).
soft_unknown is the live half. An unreadable failure now penalizes
exactly one pair (the lowest-probability hop at the attempt amount)
instead of every pair of the route in both directions, which is the
mechanism exp-019 showed spiraling lnd into give-ups from a 10%
unreadable-error rate. The smoke shows the intended signature: with
the flag on, lnd's trajectory is invariant to the unreadable rate.
adaptive_split is the measured negative, kept as instrumentation.
Three designs were built and each reduced to the same thing:
descend geometrically from the learned bound, which lnd's blind
halving already does at the fastest ratio of any variant, for free.
The supremum search paid a wire attempt per linear step; the backoff
variant re-derived halving with a slower constant; the
expected-value ladder degenerates to its top rung under apriori's
flat belief and cannot escape the retry loop bimodal pins itself
into. The conclusion the writeup carries: the reactive split-retry
control flow is not where the evolved routers' edge lives, so the
distillation question moves to plan-time mechanisms.
