test(tui): scope two whole-screen scans to what they assert - #1556

Closes #1555.

Problem

Two jcode-tui lib tests search the whole rendered screen, so text they do not control changes their result:

1. test_file_activity_scroll_reproduces_trailing_ghost_after_native_scroll_like_mutation injects ZZZZ as a ghost marker and checks !clean.contains('Z') / scrolled.contains('Z') over the full screen. The status line and header render the cwd and the random session name. A Z in either fails the pre-injection check, and makes the post-scroll check pass whether or not the injected cells survived.
2. widgets_render_detail_layer_without_repeating_status_line_facts treats every rounded box as widget text. That includes the Updates box, which lists the latest commit subjects embedded at build time. A subject containing main, OpenAI or another status-line fact fails the test.

Change

Test-only:

1. The pre-injection check reads target_line (the file-activity row the marker is injected into). The post-scroll check reads the same row of the repainted screen.
2. The Updates box is pinned empty around the draw with the existing set_unseen_changelog_entries_override_for_tests, then reset. onboarding_golden.rs and frame_flicker.rs already use the same override.

Verification (rustc 1.98.1, macOS arm64, COLORTERM=truecolor, --test-threads=1 as in CI)

(1) Before, master lib test binary run from a directory whose path contains Z:

panicked at crates/jcode-tui/src/tui/app/tests/scroll_copy_01/part_01.rs:707:
ghost marker must not be present before injection:
test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 2437 filtered out

The same binary from a directory without Z: test result: ok. 1 passed.

After, from both directories: test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 2437 filtered out.

(2) Before, on master with the Updates box holding one subject ci: keep main green (set temporarily via the override to stand in for a real build changelog):

"main" is a status-line fact and must not repeat in widgets:
 ╭─────── Updates ───────╮
 │ • ci: keep main green │
test result: FAILED. 0 passed; 1 failed

After:

$ COLORTERM=truecolor cargo test -p jcode-tui --lib -- --test-threads=1 \
    test_file_activity_scroll_reproduces_trailing_ghost_after_native_scroll_like_mutation \
    widgets_render_detail_layer_without_repeating_status_line_facts
test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured; 2436 filtered out

cargo fmt -p jcode-tui --check reports no diff in the changed lines. The file-level diffs it does report are already on master.

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