fix(update): wire up the bare-keyword mirrors mode

`update lock`, `update nothing`, and `update mirrors` were treated as
ordinary full updates: the resolver picked the highest matching
version of every root require and the install step rewrote refs from
the repository, masquerading transport metadata refreshes as content
changes (and accepting brand-new root requires the lock had never
seen). Mirror Composer's `setUpdateMirrors(true)` flow:

- Detect the bare-keyword form and skip composer.json's require /
  require-dev entirely; require each locked package by exact version
  instead. This drops fresh root requires Mozart shouldn't yet honor
  and pins existing ones to their lock version.
- After lockfile generation, walk each new entry and copy the OLD
  lock's source/dist reference back when the source/dist *type*
  matches, mirroring `LockTransaction::updateMirrorAndUrls`. URL and
  mirrors update; ref stays put — so a repo rename or mirror flip
  emits no Update operation, but a real type change (`hg` → `git`)
  still does.

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