fix(install): error on conflicting dependency directory names

[mario-eth]

No description provided.

[Copilot]

Pull request overview

This PR adds an early validation step to prevent installing/updating multiple dependencies that would map to the same on-disk directory name after filename sanitization, returning a clear error instead of proceeding into potentially conflicting filesystem operations.

Changes:

• Add a shared validate_dependency_path_collisions guard and invoke it at the start of install/update flows.
• Introduce a new InstallError::PathCollision error variant with a descriptive message.
• Add a regression test covering a sanitized-name collision case (foo/bar vs foo-bar).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
crates/core/src/update.rs	Runs the new collision validation before spawning update tasks.
crates/core/src/install.rs	Implements collision validation and invokes it in parallel and sequential install entrypoints; adds a regression test.
crates/core/src/errors.rs	Adds a new InstallError::PathCollision variant used by the validation.

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