Fix #7338: SEVERE log on shutdown when skill table re-sorts after close

[Vest]

Summary

Fixes #7338 — on macOS Cmd-Q shutdown (and any path that calls closeAllCharacters while the Skill tab is mounted), SkillRankControl.getTotalRank is invoked with a null PlayerCharacter, producing a noisy SEVERE log with stack trace. The shutdown completes correctly; the user just sees a scary trace in the log.

Root cause

CharacterFacadeImpl.closeCharacter already documents the relevant invariant explicitly:

/*
 * Unfortunately, a dummy rather than null is necessary because the UI
 * does model swaps and such that do not pause events in the UI so that
 * it is trying to update things that do not exist
 */
theCharacter = DUMMY_PC;

CharacterLevelsFacadeImpl.closeCharacter did not follow the same pattern — it assigned theCharacter = null. During teardown, dataSet.detachDelegates() fires list-change events that propagate up to the skill tree's row comparator (RowComparator.compare → SkillTreeViewModel.getData → CharacterLevelsFacadeImpl.getSkillBreakdown → SkillRankControl.getTotalRank(null, skill)). The null guard at SkillRankControl.java:64-67 swallows the NPE but emits a SEVERE record with a new Throwable() for diagnostics — that's the ticket symptom.

Fix

Mirror the parent facade's pattern: introduce a static placeholder CLOSED_PC and assign it instead of null. Empty-but-valid PC state lets mid-teardown UI events read sane defaults without triggering downstream null-guard logging.

Two-line behavioral change, scoped to CharacterLevelsFacadeImpl. Does not modify CharacterFacadeImpl or any other facade.

Test plan

• Added CharacterLevelsFacadeImplTest#testGetSkillBreakdownAfterCloseCharacterDoesNotLogError — installs a JUL Handler capturing SEVERE records, calls closeCharacter(), then getSkillBreakdown(level, skill) (the exact path on the bug's stack trace), and asserts no SEVERE record was emitted.
• On master without the fix the test fails with the verbatim ticket stack trace (Asked to get total rank for null character ... at SkillRankControl.getTotalRank(SkillRankControl.java:66) at CharacterLevelsFacadeImpl.getSkillBreakdown(CharacterLevelsFacadeImpl.java:394)).
• After the fix the test passes; the three pre-existing tests in the class also still pass (./gradlew :slowtest --tests 'pcgen.gui2.facade.CharacterLevelsFacadeImplTest').
• Manual smoke test: load a character on macOS, hit ⌘Q, confirm clean shutdown log — should be done by a reviewer with macOS access (I can't reproduce the exact event ordering without GUI access in this environment, but the test reproduces the precise call chain reported).

cc @Azhrei — would you mind retrying the macOS Cmd-Q repro on this branch when you get a chance? The unit test reproduces the same getTotalRank SEVERE log via the same call chain you reported, and stops emitting it after this change.

[Vest]

Manually verified on macOS: loaded a character, hit ⌘Q. The shutdown log no longer contains the SkillRankControl.getTotalRank SEVERE trace from #7338 — only the normal Closed character … INFO line.