[refactor] Extract install()'s per-file-type handling into installTargets.ts

[kmturley]

Summary

Stacked on #105 - this PR's base branch is fix/install-transactional-rollback, not main, since this refactor directly builds on that PR's installedDirs rollback-tracking structure (each extracted function returns the directories it populated, which install() now just folds into that Set). The diff below is only this PR's own changes; merge #105 first, then this one.

• ManagerLocal.install() was a ~200-line method mixing download/hash/elevation orchestration with the logic for where each installed file's final destination is: an installer marker directory, a Sampler's whole-directory move, an embedded .pkg/.dmg found inside an archive, format-sorted plugin files, or a flat app/preset/project move. None of these five destinations could be unit-tested without exercising the whole pipeline (real download, real hash check, real elevation branching).
• Moves each destination into its own function in new src/helpers/installTargets.ts (installInstallerFile, installArchiveFile dispatching to installSamplerArchive/installEmbeddedInstallers/installPluginFormats/installFlatDirectory), each taking an explicit params object and returning the directories it populated under typeDir. ManagerLocal.install() now just calls one of two entry points and folds the result into its rollback-tracking installedDirs Set, instead of inlining installedDirs.add(dirTarget) at each of five call sites.
• One verified, intentional behavior simplification (not a silent change): the pre-refactor code selected a formatDir from presetFormatDir/projectFormatDir when this.type was Presets/Projects, but that value was only ever read inside the this.type === RegistryType.Plugins branch - mutually exclusive with type being Presets/Projects for a single ManagerLocal instance. Those two reassignment branches could never actually be reached; formatDir was always pluginFormatDir at the point of use. Simplified installPluginFormats to just use pluginFormatDir directly, and dropped the now-fully-unused presetFormatDir/projectFormatDir imports.
• Added tests/helpers/installTargets.test.ts with 6 new unit tests, one per destination, mocking only archiveExtract (never the download/hash/elevation path) - proving each destination is now independently testable, which was the actual point of the extraction.

This is item 4 of the architectural review in review.md (Structural Warning #4).

Test plan

• New unit tests in installTargets.test.ts: installer marker dir, Sampler move, embedded .pkg/.dmg, plugin format sorting (including the "no compatible files" throw), flat directory move + exec-permission handling - 6/6 pass, raising installTargets.ts coverage to ~80% from these alone.
• All of ManagerLocal.test.ts's existing install()/uninstall() integration tests (including the new rollback regression test from [fix] Roll back partially-installed files when install() fails partway through #105) pass unchanged, confirming the extraction preserved behavior.
• npm run check (format, lint, build, test): 214/214 tests, 18/18 files pass.

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