New module cloud/routing_dryrun.py: dry-run a merged policy
document or an env mapping (file first, JSON overrides whole
tier entries, CLOUD_ROUTING_MODE overrides global mode only,
never explicit tier modes), reporting per-tier effective
model/mode/mode-source, dropped tiers with reasons, absent and
unknown tiers, and would_activate/would_shadow/would_off.
Dropped or unknown tiers block (CLI exit 1); warnings alone
(no version, fallback_models parsed but not applied, failed
file/JSON sources that the loader silently ignores) do not
block but are always printed. Model names are NOT validated
against a catalog (deliberately not whitelisted) and sample
counts are NOT checked here (that is the shadow report's job);
the CLI never writes env, files, or worker state.
Tests: test_routing_dryrun.py (8 tests).
