tooling: make upstream delta tracking reconcile-complete

[Bre77]

Intent

• The weekly upstream-drift job used total coverage-finding count as its only "did anything change" signal, so when upstream re-publishes a symbol we already carry as a local extension, schema coverage stays at zero findings and the whole reconcile path silently reported "No drift" - the pin never advanced even though the upstream file bytes had changed.
  • upstream_coverage.py now classifies each tracked upstream into four mechanical outcome classes: coverage_gap (upstream has something ours doesn't), wire_conflict (name/number/type mismatch, upstream wins names), upstream_overlap (a marked local extension that now also exists upstream, unchanged - a candidate for dropping the marker), and non_proto_commit_only (upstream HEAD moved but the tracked proto bytes didn't).
  • --dump-delta writes this as JSON, including a reconcile_needed flag per upstream and overall. That flag - not the process exit code, which still reflects schema coverage only, unchanged for both CI modes - is what decides whether a reconcile should advance.
  • A field whose upstream type is Tesla's own empty-placeholder convention (message Void {}) no longer overrides an already-concrete local field at that number; a void never replaces a concrete definition. Covered by a fixture.
  • update_upstream_pin.py only advances an upstream's commit/fetched/files when its tracked file bytes actually changed, so HEAD movement outside the tracked path prefix never produces a pin diff or a PR on its own.
  • upstream-drift.yml: the reconcile-branch/PR step now gates on the delta's reconcile_needed output instead of the coverage exit code; the "No drift" step requires both the coverage and reconcile signals to be clear. The drift issue step is unchanged (still gated on genuine coverage findings).
  • Added scripts/test_upstream_coverage.py (stdlib unittest, no new dependency): fixtures for a missing symbol, a new upstream file, a rename/type conflict, no movement, the upstream-overlap case above, and void-masking.
  • Verified end-to-end against live upstream HEAD: vehicle-command's HEAD has moved since the pin with no change under pkg/protocol/protobuf, and the new non_proto_commit_only classification correctly reports nothing to reconcile for it.
• No changeset: tooling/CI only, doesn't touch proto/ or either published package.