docs: remove Clay's deleted Ford cache and correct the Arvo state

[sigilante]

Fifth PR from the audit against urbit/urbit@08026c84b2, continuing the kernel work from #269. Two clusters: Clay's 2026-01 Ford-cache removal, and the Arvo state — which has not matched the docs for years.

Companion PRs: #266 (mechanical), #267 (Hoon signatures), #268 (user manual), #269 (Ames backpressure).

Clay: the global Ford cache is gone from vane state

Type	Change
$raft	lost fad=flow (clay.hoon:217)
$dome	lost fod=flue (lull.hoon:2524)
$flue, $mist	deleted outright — zero occurrences in either lull.hoon or clay.hoon

$pour, $soak, $leak and $flow are a different case and are treated differently. They still exist, but only inside clay.hoon's state-migration core (clay.hoon:5065-5083, sitting between the raft-16 and raft-15 definitions), where they type pre-existing state during an upgrade. They are no longer public interface — so each section is kept and marked Legacy rather than deleted, since someone reading an old pier's state still needs them.

One trap worth flagging for reviewers: $rind has a fod=(map duct @ud) at clay.hoon:275 which is completely unrelated — "reverse map of requesters to request numbers". It is deliberately untouched. A search-and-replace on fod would have broken it.

Scries /sweep and /flow are gone

The buc-x dispatch (+read-buc-x, clay.hoon:5993-6013) now serves exactly rang tomb cult esse domes tire tyre. Verified on a ship:

> .^((list [need=@ud have=@ud *]) %cx /=//=/sweep)
bail: 4

> .^(* %cx /=//=/flow)
bail: 4

/esse was undocumented and takes their place in the docs:

> .^(? %cx /=//=/esse/base)
%.y

It reports whether a desk is marked essential. The desk is mandatory — a bare /esse fails.

Arvo: the documented state has not existed for years

arvo/README.md described the payload as lac/eny/our/bud/vanes with vanes=(list [label=@tas =vane]).

The actual state is =| [_arvo soul] (arvo.hoon:1823), where $soul (arvo.hoon:215) is [mien fad=[lac=?] zen=[ver=vere lag=_|] mod=[fat lul zus van]]. Notably vanes are a (map term vane), not a list, and +$ vane is [=vase =worm].

Rewrote the section around $soul with per-field descriptions, corrected !>(..is) to !>(..part), and added a short subsection covering the larval $grub and the $heir upgrade envelope (which accepts %234 through %240). Current Arvo kelvin is %234.

Incidental link repairs

Both in files this PR already touches:

• Added a $weft section to clay/data-types.md. Two existing [$weft](#weft) links pointed at an anchor that did not exist. +$ weft [lal=@tas num=@ud] is defined in arvo.hoon:277.
• Fixed two arvo/README.md links to #solid-state-intrepeter (sic); the heading anchor is #solid-state-interpreter.

All anchor links in the touched files, cross-file and same-file, were verified to resolve. This PR was also test-merged against the four open companion PRs — no conflicts, and clay/data-types.md takes edits from both #266 and this one cleanly.

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