Make package-manager cache sharing request-scoped and configurable

[kartikjoshi21]

What kind of request is this?

None

What is your request or suggestion?

Summary

Apt and RPM/TDNF cache mounts currently use locked sharing by default. This
serializes concurrent package installations that use the same cache ID.

A configurable sharing mode would allow CI and other high-concurrency users to
choose the appropriate trade-off between safety, parallelism, and disk usage.

Proposed behavior

Add a build argument such as:

DALEC_PACKAGE_CACHE_SHARING=locked|private|shared

The default must remain locked.

• locked: safest default; serializes concurrent access.
• private: avoids blocking by using isolated cache refs.
• shared: allows concurrent access and should be documented as unsafe for
  package managers unless the user accepts possible corruption.

The setting must be request-scoped. It must not use process-global mutable state,
because multiple frontend requests may run concurrently.

Scope

• Apt cache mounts.
• DNF and TDNF cache mounts.
• Native and cross-architecture package installation paths.

Are you willing to submit PRs to contribute to this feature request?

• Yes, I am willing to implement it.