fix(organizer): preserve colliding media versions

[samtruman]

Summary

Prevent Organizer filename collisions from replacing an existing symlink
when multiple media versions resolve to the same canonical destination.

When the canonical destination is already occupied by a different source,
Organizer now preserves the existing entry and selects a deterministic
collision-safe alternate path for the additional version.

What changed

• preserve existing symlinks that point to another media source
• generate deterministic alternate names for colliding versions
• keep collision resolution stable regardless of discovery order
• keep repeated Organizer runs idempotent
• never replace or remove real files/directories during collision handling

Tests

Added dedicated Organizer collision tests covering:

• multiple versions resolving to the same canonical destination
• deterministic collision naming
• discovery-order independence
• idempotent repeated runs
• protection of existing symlinks
• protection of real files and directories

Validation:

• Organizer tests: 14 pass
• full suite: 88 pass
• typecheck: pass
• backend build: pass
• git diff --check: pass