fix(scheduler): extend core-isolation nm-audit to core_events.c/core_session.c/core_clock.c (Req 5 AC 1/2)

test_core_isolation.c's build-time proof (nm(1) undefined-symbol scan of
sched_core_isolation_lib) previously covered only core.c -- core.c's own
800-line-cap siblings, holding scheduling logic just as boundary-sensitive
as core.c itself, were "isolation-clean by review" only, with nothing
mechanical catching a regression. Extends sched_core_isolation_lib to
compile all four files together, and the source-text scans (forbidden
headers/symbols, wall-clock calls) to also cover core_events.c/
core_session.c. core_clock.c joins only the nm(1) layer, not the
wall-clock text scan -- it's the one sanctioned exception carrying the
core family's single legitimate clock_gettime() call.

src/scheduler/testsuite/meson.build

@@ -208,6 +208,12 @@ sched_core_isolation_test = executable(
              '-DXCE_CORE_C_PATH="'
              + (meson.current_source_dir() / '..' / 'src' / 'core.c')
              + '"',
+             '-DXCE_CORE_EVENTS_C_PATH="'
+             + (meson.current_source_dir() / '..' / 'src' / 'core_events.c')
+             + '"',
+             '-DXCE_CORE_SESSION_C_PATH="'
+             + (meson.current_source_dir() / '..' / 'src' / 'core_session.c')
+             + '"',
              '-DXCE_CORE_LIB_PATH="'
              + sched_core_isolation_lib.full_path()
              + '"'],