A declared browser session is stored the first time a run reaches for it

[czpython]

What

A BrowserSession declared on an extension had no way to reach its stored row: the identity is namespaced (x_me.x), and the only code path that creates a row is POST /api/browser-sessions, whose name pattern accepts neither dots nor underscores. Every borrow of a declared session raised BrowserSessionUnknownError, so the author surface documented in writing-an-extension.md could not run.

The declaration now resolves its row on first use, the way a Service gets its ServiceIdentity on connect — nothing is written at import or install. The first borrow writes the row as needs_login with the declared site, which is what puts it in the sessions pane for the operator to sign into; the borrow itself still refuses, now saying the session is not ready rather than that it does not exist. mark_stale() resolves the same way, so reporting a bounced login before the first successful borrow lands on a row instead of raising.

Testing

• The first borrow of a never-used declaration writes a needs_login row carrying the declared site, raises not-ready, and a second borrow reuses that one row rather than writing another.
• Existing borrow coverage (ready sessions, writer-lock contention, launch failure, persist write-back) is unchanged.