When connected to the user's real Chrome via the extension relay, sequential
commands could land on the wrong tab: `get url` returned x.com/home, then with
no navigation in between `eval` executed against x.com/notifications — so it
read the wrong page and returned nothing.
Root cause: the session's anti-drift anchor is `active_target_id` (pinned by
stable target_id), documented to be set "on every explicit open". But `open`
runs through `navigate()`, which never pinned. On the relay path `open` reuses
an existing tab via `navigate` rather than `add_page` (the only "explicit" path
that pins), so `active_target_id` stayed `None` and the session rode the fragile
`active_page_index`. A later passive tab close/reorder (drained before every
command) then drifted `eval`/`get url`/`snapshot` onto a foreign tab.
Fix:
- `navigate()` now syncs the index to the resolved active page and pins it by
target_id after a successful navigation — restoring the "pin on explicit open"
invariant for the relay path.
- `ensure_page()` pins its freshly-created tab too (matches `add_page`).
- Extract the pin-vs-index resolution into a pure `resolve_active_index()` and
cover the invariant with unit tests (pin beats stale index; falls back when
the pin is gone; survives passive background-tab discovery).
cargo fmt + clippy -D warnings clean; full suite 816 passed.
