[Repo Assist] test: add 10 unit tests for switch_view state reset and scrollbar_jump

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What

Adds 10 unit tests covering two previously-untested functions in handler.rs:

switch_view state reset (6 tests)

PR #156 tested that Left/Right keys change app.mode, but did not verify the side-effect state resets that switch_view performs. These tests close that gap:

Test	What it verifies
switch_view_clears_local_filter	local_filter is emptied so the new view starts unfiltered
switch_view_clears_selected_packages	Multi-select indices are dropped (they are invalid in the new view)
switch_view_clears_detail_panel	Stale detail from the old view is not carried over
switch_view_increments_detail_generation	Generation counter is bumped to discard in-flight detail requests
switch_view_resets_focus_to_package_list	Focus returns to the package list after switching
switch_view_no_op_when_already_on_target_mode	Switching to the already-active mode is a no-op

scrollbar_jump position math (4 tests)

scrollbar_jump converts a pixel row to a list index via a ratio calculation. It had no test coverage at all:

Test	What it verifies
scrollbar_jump_selects_first_item_when_clicking_track_top	Top of track → item 0
scrollbar_jump_selects_last_item_when_clicking_track_bottom	Bottom of track → last item
scrollbar_jump_no_op_on_empty_list	Does not panic or mutate state when list is empty
scrollbar_jump_clamps_row_below_track_top_to_first_item	Row above track is clamped to item 0

Why

The switch_view invariants (clear local filter, clear multi-select, clear detail, bump generation) are important for correctness. A regression in any of them would cause confusing UI state when the user presses Left/Right to switch views. The scrollbar_jump ratio calculation is non-trivial and benefits from boundary-condition tests.

Test Status

cargo check --all-targets  → clean
cargo fmt -- --check       → clean
cargo clippy -- -D warnings → clean
cargo test                 → 241 passed (was 231) — 10 new tests

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