feat(workspace): replace markdownMaterializer with createMaterializer factory

[braden-w]

The markdown materializer started life as a single monolithic function that owned serialization, file writing, and Yjs observation all at once. That was fine while the API was still being figured out, but the spec kept moving—closure-based document access, opt-in materialization, factory composition—so the implementation had to stop pretending the shape was settled.

This PR follows that path and lands on a workspace-bound createMaterializer(...) factory. The materializer now plugs into the extension chain, waits for whenReady before reading state, and lets each table or KV namespace opt in explicitly instead of forcing everything through one big markdown path.

// before: one function did everything
markdownMaterializer({ directory: './data', tables })

// after: compose the materializer at the workspace boundary
.withWorkspaceExtension('materializer', (ctx) =>
  createMaterializer(ctx, { dir: './data' })
    .table('posts', { serialize: slugFilename('title') })
    .kv(),
)

While getting there, the repeated “compute once inside an observer” pattern got pulled into a reusable lazy() helper so those caches stop being hand-rolled in three different places.

const getAllEntries = lazy(() => yarray.toArray())

const entries = getAllEntries()

That cleanup also exposed a real scaling bug in YKeyValueLww: bulk updates were re-scanning the array on every write, which made the hot path quadratic. The fix batches the conflict cleanup so the observer resolves the batch once instead of repeatedly walking the same data.

There’s also a short article on the lazy initializer pattern, plus doc updates to keep the workspace docs aligned with the current API instead of the older chaining examples.

Stacks on #1649. 13 commits, 14 files changed, +1322/-438.