Add chunked paste support for content up to 1.44 MB

Large pastes are split into 8 KiB chunks on the client side,
each stored separately in a dedicated chunks/ directory.
A version-2 manifest paste lists the chunk hashes and is
announced to the DHT; chunks replicate via periodic republish
with per-put throttling to avoid rate-limit bans.

- New PUTC/PUTM protocol commands for chunks and manifests
- Client-side chunking avoids O(n^2) base58 on large content
- HTTP handler reassembles chunks directly from store
- DHT sync routes incoming chunks to chunks/ directory
- Republish interval reduced to 5 min with 200ms throttle
- tp.1 updated with new 1.44 MB limit

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