Raise the page and jail byte caps to 8 MB

[SecureCloudGroup]

Per the 2026-10-09 ruling: the Neural Interface will read bigger pages once the page browser lands, and 2 MB refused long schedules, rankings and script-heavy pages.

• netguard static page/text/JSON fetch cap: 2,000,000 → 8,000,000 bytes
• extractor jail input cap (bytes the parent writes to the child): 2 MB → 8 MB
• extractor jail hard stdin cap inside the child: 4 MB → 16 MB
• extractor jail stdout cap: 1 MB → 4 MB
• docs (ni-format.md §15/§16) and the page-graph test bound updated; comments brought current

The memory bound stays the real protection (RLIMIT_AS 768 MB on Linux; the macOS gap G1 is unchanged and tracked in the Round 20 plan).

Gate (Docker, smartbrain_3000:dev): ruff clean; targeted jail / netguard / pagegraph / page_verify / ni_flow_frame_verify = 535 passed; full suite = 4,072 passed, 37 skipped.