[shovon]

Closes #132. Part of #84.

What changed

Page properties (packages/pages/properties.ts, new): the bottom layer of #84. Nothing writes these events yet and nothing shows them; the builders and commands are #133.

• Five events, each under NAMESPACE at v: 1: page.property.set, .unset, .item.added, .item.removed and .removed. Payload types with doc comments on what each field means.
• The fold runs in two steps:
  • parse turns a payload into edits to single paths, or skips the event whole if it is malformed (ledger: ignore what the format doesn't name, instead of refusing or reporting it #104).
  • apply makes the edits. An entry left with nothing is deleted, then a page left with no entries.
• removed is several of the edits the single events make, applied in one go. It clears a seen value only if it is still the one seen, and the items it lists. What another device wrote that the remover never saw survives, in either fold order.
• Normalizing: the fold trims path segments and puts every string in NFC. Scalars are strings, finite numbers, booleans and null, and keep their type. -0 is 0.
• No timestamps: the fold reads { type, v, payload } like the block fold, and keeps no at.
• Reads: get, getValue, getItems, list, children, and the path / showPath helpers. Sorted by path segment, by codepoint. Every result is a snapshot.

The pages fold (pages.ts, blocks.ts):

• Pages gains properties beside blocks and titles.
• pagesFold runs the block fold, then the property fold. Each hands back the state it was given for an event that is not its own.
• HANDLES is the block table plus the property table. The block half is BLOCK_HANDLES inside the package; the public HANDLES keeps its name.
• getPages lists a journal day that has properties and no blocks.
• A property event spells its page's title the way block.created does; spell is exported within the package for that.

README.md: a "Page properties" section with the five events, the rules, and the reads.

For the reviewer

• A fix the new field needed: block.created built its result from blocks and titles alone, which would have dropped properties. It now spreads the state.
• One test narrowed: "every builder writes the v its own gate reads, and there is one for every event" now checks block events only. The property builders are Page properties: builders and commands #133, which should restore the full check.
• Live vs reopened: the app folds a write's payload in memory before it goes through JSON. A NaN would be skipped live but written to disk as null, a value, and show after a reopen. The fold can't tell; Page properties: builders and commands #133's commands refuse non-finite numbers for that reason.
• Export names: get, list and children are top-level package exports, as Page properties #84 names them. Happy to rename to something property-specific next to getPage if preferred.
• The ledger is untouched. No format change; a build without this skips the property events through its own gate.

Tests

• properties.test.ts (20 tests, new), folding through pagesFold:
  • each event's effect; unset leaves items alone and item.removed leaves the value alone
  • the later write in fold order wins for the value and for each item; two writers adding different items keep both; a re-added item moves to the end
  • emptied entries, parents and pages are gone, and list and children don't show them
  • a page with properties and no blocks is a page, under the first spelling seen
  • malformed payloads are skipped whole; the gate skips a newer v; __proto__ as a title, a segment or an item is an ordinary key
  • NFC and NFD are one segment, value and item; 1, "1", true and null are four items; trimmed segments; "" is a value
  • removed: what it lists is cleared, what it doesn't list survives in either fold order, null and an absent value
  • list and children on nested paths, the root, a scope that holds something itself, ["foo"] beside ["foobar"], codepoint order
  • reads are snapshots; the fold never changes the state it is handed; an event that changes nothing hands back the same state
• Existing tests: blocks.test.ts builds Pages with properties and checks BLOCK_HANDLES.
• npm test --workspaces (233 pass), npm run lint -w app and npx tsc --noEmit -p app are clean.

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