nip44: allow encryption of payloads larger than 65535 bytes

Extend the v2 padding format with a backwards-compatible sentinel:
when the first 2 bytes of the length prefix are zero, the next 4
bytes encode the plaintext length as a big-endian u32. This raises
the maximum from 65535 bytes to 2^32-1 bytes without requiring a
version bump.

Fixes from nostr-protocol/nips#1907:
- Fix off-by-one: use >= 65536 (not > 65536) for the extended path,
  since u16 can only represent 0..65535
- Fix padding validation: use dynamic prefix_len (2 or 6) instead of
  hardcoded 2 in the unpad() size check
- Fix len(d) typo in decode_payload (should be len(data))
- Remove upper-bound size checks in decode_payload that would reject
  large payloads
- Add write_u32_be, read_uint16_be, read_uint32_be to function list
- Add extended_prefix_threshold constant
- Update size range comments for both small and large payload paths

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