fix(scheduler): close core_clock.c isolation-test coverage gap (bug-1827)

core_clock.c joined the mechanically-enforced isolation archive
(sched_core_isolation_lib) and its own doc comment claimed coverage by
the forbidden-headers/forbidden-symbols text scans, but was actually
absent from both -- only the nm(1) undefined-symbol layer covered it,
which cannot catch a forbidden #include resolving to no undefined
symbol (a macro-only header).

Added XCE_CORE_CLOCK_C_PATH (meson.build, mirroring the existing
per-file path macros) and extended both
test_core_never_includes_forbidden_headers() and
test_core_never_calls_forbidden_symbols() to scan core_clock.c. It
remains excluded only from the wall-clock scan, for its one sanctioned
clock_gettime() call.

TDD: confirmed RED by temporarily inserting a forbidden #include into
core_clock.c (84/85, correctly attributed failure), reverted
(byte-identical to HEAD), reconfirmed GREEN (85/85). No live violation
existed -- this closes a coverage gap in the mechanism itself.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RLSNJhNoULvVcUhXEFafQC

src/scheduler/testsuite/meson.build

@@ -231,6 +231,9 @@ sched_core_isolation_test = executable(
              '-DXCE_CORE_SESSION_C_PATH="'
              + (meson.current_source_dir() / '..' / 'src' / 'core_session.c')
              + '"',
+             '-DXCE_CORE_CLOCK_C_PATH="'
+             + (meson.current_source_dir() / '..' / 'src' / 'core_clock.c')
+             + '"',
              '-DXCE_CORE_LIB_PATH="'
              + sched_core_isolation_lib.full_path()
              + '"'],