Add size validation for OP_SERVERMESSAGE packets - #447

…e-project#431) (amule-project#447)

Collapse hits that are the same file (same ed2k hash AND size) but
advertised under different filenames into one expandable parent row,
end to end, and let a chosen alternative name be downloaded.

Phase 1 - expose grouping:
- Core (ExternalConn.cpp): the EC_DETAIL_FULL search responder now
  descends into GetChildren() and emits each child when the caller opts
  in with an empty EC_TAG_SEARCH_PARENT flag on the request. amulecmd /
  amuleweb don't send it, so their flat parents-only list is unchanged;
  the INC_UPDATE (amulegui) path already emitted children. Children
  carry their parent's ECID via EC_TAG_SEARCH_PARENT (already in
  CEC_SearchFile_Tag).
- webapi: RefresherTick sends the flag; ApplySearchFull reads
  EC_TAG_SEARCH_PARENT and folds children into the parent's children[]
  (name/hash/ecid/sources), dropping them from the top-level set;
  WriteSearchObject + the search_result_added SSE emit children[]
  (always present, empty for a single-name hit).

Phase 2 - download under a chosen name:
- Since grouped children share the parent's hash, hash alone can't pick
  one. POST /search/results/{hash}/download takes an optional "ecid"
  (a child's ECID) that rides as an EC_TAG_SEARCHFILE selector;
  CSearchList::AddFileToDownloadByEcid resolves it across parents +
  children and downloads that specific result, so the partfile lands
  under the chosen filename. Omitted => the parent, unchanged.
- The download responder now reads the category by name
  (EC_TAG_PARTFILE_CAT) so the ecid selector can ride alongside it.

RefresherTest.SearchResultGroupingFoldsChildren; curl 19 children[]
shape + ecid 400; REST + SSE docs. Verified live on a connected daemon
(search 'euphoria': 3 grouped parents / 6 children; download 202,
ecid-type 400). Frontend rendering is deliberately out of scope.