Add the routing quest, and explain fees where they belong

[hotpixelgroup]

Quest #4 showed a payment crossing hops without ever saying where the route came from or who decided what it cost. Routing read as magic; fees were a passing clause.

Quest #6 — "How does your wallet find a route?"

Four stops, deliberately separating the two questions people conflate:

• There is no map — gossip, and why a channel announcement can't lie (it names the on-chain output backing it)
• How a fee is SET — each node picks two numbers per direction and advertises them. No schedule, no floor, no approval
• How a fee is ASSESSED — one formula, integer truncation, charged on the amount forwarded, paid entirely by the sender in advance so the payee always receives the invoice amount
• Choosing a path — cheapest ≠ shortest, and why liquidity is never gossiped (which is why payments fail and retry rather than being quoted)

BOLT #7 works its own example through a four-node network and prints the answer, so the finale reproduces the spec's own sum and won't claim anything unless it matches to the millisatoshi.

A real bug, caught by the spec

My first implementation charged each hop using the channel the payment arrived on. The spec settles it in the worked example — A pays B the fee it specified in the B->C channel_update, the channel it forwards out of — and my arithmetic came out at 5,099 instead of 10,199.

Testing against a primary source caught what testing against myself never would have. A consequence worth stating in its own right: the first channel of any route is free, because the node forwarding out of it is the sender, and a sender doesn't charge itself.

Also

• Six glossary entries for the fee/gossip vocabulary (Routing fee, fee_base_msat, fee_proportional_millionths, channel_update, Gossip, short_channel_id)
• Two figures: a fee calculator that demonstrates the truncation, and a route race showing two-hop paths differing 2× on price alone
• Quest Bump js-yaml from 4.3.0 to 4.3.1 #4 now points forward instead of leaving its fee clause hanging
• whose-lightning renumbers to Signpost each site from the other, clearly and in both directions #7 so routing sits inside the Routing track

Verification

147 tests green (39 in lightning-logic); lint, build, typecheck clean. Exercised in-browser: spec check returns 10,199, routes rank A→B→C (2.2 sats) over A→D→C (4.4 sats), first hop shows no fee.

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