Generalises the importer from one hardcoded browser to a source registry with
two engines.

Chromium forks are table entries: Chrome, Edge, Brave, Vivaldi, Opera, Arc and
Helium all share the existing extractor and differ only in their paths and
keychain coordinates. Those coordinates are pinned per fork rather than
derived, because the forks disagree — Helium uses "Helium Storage Key" /
"Helium" where the others use "<Name> Safe Storage" / "<Name>".

Firefox is a second engine. It stores cookies unencrypted in `cookies.sqlite`,
so there is no key to fetch and no consent prompt — that is Mozilla's design
choice, not a control being circumvented, and it is why Firefox works
identically on all three platforms while Chromium still needs a per-platform
credential store.

Paths resolve for macOS, Windows and Linux from an injected context rather
than from `process`, so a platform's layout can be checked without running on
it. Chromium off macOS still reports `unsupportedPlatform` until those key
stores land; Firefox does not.

The snapshot-before-read step moves to a shared module, since both engines
keep the database open with WAL and must never have the browser's own file
opened for writing.

Firefox has tests against a real `moz_cookies` fixture, including that the
source file is left untouched, and `profiles.ini` parsing covers the
`Install*` sections that name a default profile without describing one.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>