Pages exist: stop saying they don't, and allow page-targeted events

[shovon]

The code and docs currently say that pages don't exist: "a page is not a record anywhere", "the block is the primitive", "pages come into being by being named". That was never quite true. The fold is keyed by page title (Pages = Record<string, Block[]>), and the title is a page's identity. Now it's also getting in the way: page properties (a table opened from the page header) want an event that targets a page, and the current wording reads as a rule against one.

What stays true

• There is no file per page. Everything is in the log.
• Pages are never created. Naming one, by a link or by the calendar, is enough, and opening a page appends nothing.
• Block events after block.created are addressed by block id alone. That rule has its own reason, independent of whether pages exist (see app/docs/block-sync.md:29), and it stays.

What changes

An event that explicitly targets a page (e.g. github.com/shovon/pbnotes#page.property.set { page, key, value }) is allowed. A page is a first-class concept identified by its title. It has no lifecycle events, but it can have events of its own.

Wording to rewrite

• app/docs/design.md:68: "A page is not a record anywhere. It is the set of blocks carrying its title…"
• packages/pages/blocks.ts:1-9: "The block is the primitive… A page is not a record anywhere: it is a title that blocks carry, so pages come into being by being named and go away by having nothing left on them."
• packages/pages/blocks.ts:93-97 (BlockCreated): "The only event that carries a page, because it is the only one that says anything about one". It should say this is the only block event that carries a page.
• packages/pages/pages.ts:201-203 (searchPages): "naming a page is all it takes for one to exist". Fine as is, but check it against the new wording.
• app/src/shared/pages.ts:8-14: "there is no page record anywhere, so a page is never created".

Open questions this unblocks

• Does a page that has properties but no blocks still exist? getPages currently drops a journal day that has no blocks.
• Should links in a property value count as references in getReferences and searchPages?
• Are property keys case-sensitive, like titles are?

[shovon]

It's being worked on.