fix(history): satisfy imported window clippy

[beruro]

Problem

The current develop branch fails strict Rust Clippy in imported-history window construction because a nested else { if let ... } triggers clippy::collapsible_else_if under -D warnings. This blocks Rust CI for every branch rebased onto develop.

Solution

Collapse the nested conditional into else if let while preserving all three existing paths: loaded turns retain full chunks, unloaded indexed turns retain previews and placeholders, and missing turn metadata retains the original user chunk while discarding its orphaned body chunks.

Potential risks

The change is a control-flow-only rewrite with no data-format, persistence, API, dependency, concurrency, or UI changes. The related imported-history tests cover the affected window behavior. Rollback is a normal revert of this commit.

Verification

Completed again on published head 8b5dc34e6, rebased onto develop at 9a2d6f6cf:

• rustfmt --edition 2021 --check crates/orgtrack-core/src/sources/imported_history/window.rs — passed.
• cargo test -p orgtrack_core imported_history --lib — passed, 72 tests; 2 existing benchmark/integration tests ignored.
• cargo clippy -p orgtrack_core --all-targets --no-deps -- -D warnings — passed.
• git diff --check — passed.
• Final scope inspection — passed; the diff contains only src-tauri/crates/orgtrack-core/src/sources/imported_history/window.rs. Two unrelated UI commits introduced by an automated branch update were removed before publication.
• cargo fmt --check --all was also attempted but is currently blocked by unrelated pre-existing formatting drift elsewhere on develop; this PR does not include that repository-wide churn.
• Current GitHub frontend job — typecheck and lint passed; full Vitest executed 1,022 files / 8,266 tests and failed only the four unrelated Markdown SSR baseline cases isolated in test(markdown): stabilize issue SSR mocks #753 (1,020 files / 8,262 tests passed). The macOS Rust job is queued; targeted strict Clippy passed locally as recorded above.
• UI screenshots were not captured because this backend control-flow cleanup has no rendered behavior change.

[beruro]

Merge-order note: this PR fixes the current develop Rust Clippy baseline; #753 independently fixes the current develop frontend test baseline. The current frontend job here passed typecheck and lint, then failed only the four tests covered by #753. After either baseline PR lands, the remaining PR should be rebased and rerun so both required jobs become green.