What happens when the Target page changes

[stueydubs]

Question

What happens when the Target page changes, and what is the Target when none was named?

Split out of #5, which defined a Project as holding many pages with exactly one Target page at a time, and then never said how that one is chosen or what changing it costs.

CONTEXT.md says a Target page has an Overlay, an Edits file beside it, a pending Batch that reconcile flips, an SSE connection, and a watcher over the served tree. All of that is written for a process that opens one page and exits.

Resolve:

• What opens for webtweak or webtweak site/, where no page was named. "No detection" was settled on What a Project is #5, which rules out silently preferring index.html, yet the relationship claims exactly one Target page always exists.
• Whether a Project may legitimately have no Target page, which is what a picker with nothing selected implies.
• What happens to unsaved edits when the Target page changes. The reload rules already guard on dirty, and rule 3 forbids a reload while this session's Batch is still pending, because reconcile writes source before marking the Batch. A page switch is a new way into that same window.
• Whether the watcher and the SSE connection are torn down and rebuilt per page, or span the Project.
• Whether the viewport stamp and Batch identity still mean anything across a switch. One edits file per page, or one per project #9 owns the file layout; this ticket owns the session semantics.
• Whether a page excluded from membership can still be opened directly, and what that implies for "a Target page is a page in the Project".

[stueydubs]

Amended by the runtime change

Still a live question, and its shape is unchanged: what happens to unsaved edits, a pending Batch, and the live-reload machinery when the Target page changes.

Two of the four things this ticket asks about - the watcher and the SSE connection - are server machinery that will not exist. Whatever replaces them is #22, so the teardown question here should be asked of that replacement rather than of fs.watch and SSE.

Also settled by the now-closed #16: page switching within one Project is not a privilege boundary, since anything that can request the switch can already reach every file in the Project. So this ticket is purely about correct teardown and user-visible data loss, not about authority.