MCSS_ namespace hygiene: prefix should mean runtime_context-owned

[aradanmn]

The MCSS_ prefix already exists on main with mixed meanings: MCSS_GEOM_DIR (genuinely cross-module), MCSS_VERSION/COMMIT/BUILD_DATE (provenance), MCSS_MIN/MAX_MEM_MB (installer), but ALSO window_manager-local tunables MCSS_SKIP_UNCHANGED, MCSS_REASSERT, MCSS_REASSERT_DELAY_S (window_manager.sh:364-383) and the tests' MCSS_MODULES override (tests/kwin-diag.sh:21, tests/kwin-place-test.sh:21).

Once the globals centralization lands, MCSS_ should mean "runtime_context-owned cross-module global" so contributors don't mistake module-local knobs for canonical globals:

• Rename window_manager's module-local MCSS_SKIP_UNCHANGED / MCSS_REASSERT / MCSS_REASSERT_DELAY_S → WINDOW_MANAGER_* (keep old names as accepted aliases for one release, or do a clean break — they are test/debug knobs)
• Decide MCSS_MODULES: either the path resolver honors it as the modules-dir override, or the two tests migrate to $MCSS_SCRIPT_DIR/modules
• Add readonly+export to MCSS_VERSION/MCSS_COMMIT/MCSS_BUILD_DATE after fallback normalization so children log the same provenance (single-derivation hygiene item from the globals review)

Plan ref: Part 4 "Namespace hygiene".