docs(design): add company-to-corfu migration spec

Replaces a thin third-party config snippet (one use-package corfu + one
use-package cape, with no migration steps and no prescient piece) with
a full spec covering the current company stack: corfu, cape,
corfu-popupinfo, kind-icon, corfu-prescient.

Maps every current company setting to its corfu equivalent
(idle-delay, prefix-length, tooltip-limit, wrap, require-match,
global-mode exclusions, doc popups, icon kinds, prescient sort).
Walks the per-module fixups -- selection-framework, mail-config,
ledger-config, latex-config, eshell-config, and the three prog-*
mode hooks.  Adds a test plan and risks section.

todo.org points at the new doc; the broken :COMPLETE_CONFIG:
property (which referenced the wrong line range in someday-maybe)
is gone.

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