fix(newos4): stop reporting network and VFS size as facts we do not have (v4.6.161)

1. The last two `navigator.onLine`-as-fact payloads
   - module-aiassist.js: the debug/health payload carried
     `online: navigator.onLine`, consumed by downloads/dashboard.html, which rendered
     "🔴 Offline" — on an Android WebView (reports false while fully connected) that
     told working users they were offline. The key is gone; the payload carries
     `network:'unknown'`, `networkKnown:false` and `networkHint`.
   - module-world-model.js: the snapshot had the same field, and it is also handed to the
     LLM through `_modelDigest()`/`ask()`, so the model was told the user was offline.
     Now `network:'unknown'` + `networkKnown:false`.
   - `_loadFromVFS()` scrubs `system.online` from a model persisted by an older build —
     otherwise the stale boolean would survive inside the digest for the life of the VFS.
   - downloads/dashboard.html: `networkText()` renders reachable/unreachable/http-* only
     when doctor produced them ("(doctor)"), `unprobed` as "— not probed", and everything
     else as "— unknown (run doctor for a live probe)". A legacy payload's `online:false`
     lands in "unknown", never "Offline". The row label is "Network", not "Online".
   - Neither payload invents a probe: SystemModule stores no last-doctor verdict, so
     'unknown' is the honest value until one exists.

2. `vfsSize` in both payloads was a different unit again
   - aiassist: `JSON.stringify(fs.root).length / 1024`; world-model:
     `JSON.stringify(fs.root).length`. Same lie v4.6.160 fixed in `vfs-size` — UTF-16 code
     units plus serialisation structure. Both now sum UTF-8 bytes of file content
     (`_utf8Len`/`_measureVfs`, Blob → TextEncoder → code-point fallback, depth/node
     capped) and carry `sizeBytes` beside the human string so the rows reconcile.
   - world-model's `_gatherFileInfo().size` used a THIRD unit on an adjacent line
     (`JSON.stringify(fs.root).length/1024`); now the same measurement, plus `sizeBytes`.
   - `showModel()` renders exact bytes and tolerates a pre-fix persisted string.

3. Predictor made a guaranteed-401 request on every boot (UI-plan finding F6)
   - `_wdConfig()` returns `user:''`/`pass:''` when the saved WebDAV config has a URL but
     no credentials, and `_pullFromServer`/`_pushToServer` fetched the auth-gated
     `/spaces/<user>/_data/predictor.json` anyway — 107 × 401 in one day, logged by the
     browser as console errors. Both now refuse without credentials
     (`cache-only (no credentials)`).

Verified: 42/42 node tests, 2 new files (38 payload/honesty assertions, 8 predictor-store
assertions); the predictor suite is 4/8 against the pre-fix source, and the payload suite
fails 14 assertions against HEAD (`online:false` present, vfsSize === JSON.stringify
length). Live Chromium on v4.6.161, on the served dashboard: a legacy payload with
`online:false` renders "Network — unknown (run doctor for a live probe)", the current
payload renders "85.8 KB (87880 B)" and the same unknown, and a doctor-produced
`unreachable` renders "⚠️ unreachable (doctor)". Served bytes identical for
core-bundle.js, main.html and downloads/dashboard.html. `maintain.py --fix` all PASS.

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