refactor(orgtrack): remove unused imported history modules

[beruro]

Problem

Three tracked files under sources/imported_history duplicate 600 lines of the live implementation in mod.rs, but the parent module never declares or imports them. Because Rust never compiles these files, they form a second, misleading copy of core imported-history behavior and can silently drift from the production path.

Solution

Remove the unreferenced chunks.rs, impact.rs, and rows.rs files. The authoritative implementation remains in sources/imported_history/mod.rs, which is the path used by all production callers and the existing imported-history tests.

Potential risks

Runtime risk is low because the deleted files were not part of the Rust module graph and had no production callers. The main risk would be an undocumented external build step reading source files by path; repository-wide reference and module-declaration sweeps found no such path. Rollback is a normal revert of this commit.

Verification

• cargo check -p orgtrack_core — passed before and after the deletion.
• cargo test -p orgtrack_core imported_history — 72 passed, 2 ignored, 0 failed.
• git diff --check — passed.
• cargo clippy -p orgtrack_core --all-targets -- -D warnings — attempted; blocked by a pre-existing collapsible_else_if warning in unchanged sources/imported_history/window.rs:189 on develop.
• Confirmed there are no mod chunks, mod impact, or mod rows declarations for the deleted files and no production imports of those modules.

UI evidence is not applicable because this removes Rust source files that were never compiled or rendered.

[beruro]

Superseded by #780, which preserves this change as an independent commit and has passed the consolidated CI suite. The source branch is retained for recovery.