Keeps the upgrade host's step handler map

[johnnyt]

Summary

On every cancel, the upgrade host's UpgradeHost.Loans.handler_for_invocation/1 found an invocation's handler by authoring the loan document again (the palette and every gated edit, through LoanDocument.document/0) and reading the palette, although the compile it maps the invocation's state through was already kept per node.

The block-to-handler map (a step block's id to the handler serving its invoke type) is now built once per node from the document and the palette, kept under :persistent_term beside the compile through the module's existing cached/2, and the lookup maps the provenance's block id through it. The build loads each palette module before asking it for invoke_type/0, because the map is kept and a miss would be kept with it.

Every answer is unchanged: a state with no owner in the provenance, a block that is not an invoke step, and an invoke type no handler serves each still answer :error.

Tests

A new loan test, "the handler lookup reads each step's handler from a map kept once per node, not from the document", pins that the map is kept after a lookup and that the lookup answers from the kept map (a swapped map changes its answer; the test restores the kept map on exit). The existing loan test's two lookups and its unknown-state :error stay as they were.

Sabotage, both red on an assertion and restored byte-equal: the map built on every call with nothing kept (the kept-map assertion read nothing); the lookup dropping and rebuilding the map on every call (the swapped-map assertion answered the fine's real handler).

Gate

The root mix quality and, from upgrade_host/, the CI job's mix deps.get --check-locked, mix compile --warnings-as-errors (test env) and mix test all green on the rebased head. The upgrade host's exact pins in upgrade_host/mix.exs and its mix.lock do not move.

Review

The pre-request review round (a diff critic and a test critic) found nothing blocking. No record text in this change.