fix(workspaces): stream kernel file reads to avoid Durable Object resets

[posthog]

Why

A user hit two full-page crashes in one workspace session: the WorkspaceKernel Durable Object kept blowing past Cloudflare's isolate-memory and storage-operation-timeout limits and resetting itself, which dropped the client's live connection and blanked the whole page. Both flavors appeared in the same session — a storage-operation timeout, then an isolate-memory reset right as the user opened a PDF, exactly the large-file case the buffer-everything design couldn't handle.

What changed

• Stream file reads instead of buffering. readFileContent loaded whole blobs into a single in-memory Uint8Array inside the DO isolate. The content route now uses a new streamed read (readFileStream) so R2-backed bytes flow straight through and are piped directly into the Response, never materializing the full file in the isolate.
• Keep large blobs out of inline DO storage. Lowered the inline SQLite threshold from 1.5MB to 128KB so anything substantial spills to R2 rather than bloating a single SQLite row (base64 inflates ~33%), which was the source of the storage-operation timeout. This only governs new writes — existing files keep whatever backend they were written with.
• Safety net. Wrapped the workspace file viewer in a scoped error boundary so a transient content-load failure (e.g. a DO reset mid-request) degrades to a pane-level message with Retry instead of bubbling to the root boundary and taking the page down.

Notes / verification

• vp check (format + lint + typecheck) passes.
• The streamed read returns a ReadableStream across native Durable Object RPC (the agents/partyserver stub is a native DurableObjectStub), so bytes transfer without buffering in either isolate.
• The background extraction workflow still reads full bytes (readFileContent) because OCR/extraction inherently needs the whole file; that path runs in a retrying Workflow and is not the reported crash. Left unchanged to keep this focused on the file-viewing flow.
• The error boundary is a defense-in-depth addition; its runtime UX could not be exercised in this environment (no live Cloudflare runtime), but the root-cause streaming + threshold changes are what stop the DO from resetting.

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Created with PostHog Code from an inbox report.

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[github-actions]

React Doctor found no new issues. 🎉

_{Reviewed by React Doctor for commit d2e91d1.}