v0.31.3 — an agent can find its own tabs on the fallback

On the raw-CDP fallback, an agent can find its own tabs again

wb close --agent <name> matches on the [1-2] <agent> title tag. The fallback's createTab
never applied one, so tabs an agent opened on that path were invisible to it: wb close answered
"no open tabs" and closed 0 while the tabs sat open on screen. Reported 2026-10-03 — an agent
left five tabs behind rather than risk closing a person's.

The tag script now lives at module level and both lanes use that one source, so they cannot
drift into tagging different things. Same failure shape as 0.31.2's dead-tab cleanup, which lived
only in rawcdp.js and therefore ran only when the engine was already degraded: behaviour that
differs by code path, where the untested path is the broken one.

Two details that were wrong on the first attempt and are now pinned by tests:

• It stamps after the page settles. Stamping right after createTab reported stamped while
  the title stayed bare — the tab is about:blank or mid-navigation then, and the navigation
  replaces the document, taking the title with it.
• It reads the title back before claiming success. The first version pushed stamped
  unconditionally and was wrong on the very first run. When the tag does not take you now get
  stampWarning saying so, and what it costs: wb close --agent <name> will not find that tab.

Verified end to end on the fallback: done:["newtab","stamped"], title
[1-?] stamp-probe2 Example Domain, then wb close --agent stamp-probe2 → Closed 1 tabs,
leaving an untagged tab on the same URL untouched.