[ktf]

Follow-up to #16019, from a review of the reader:

• Refuse leaves with fLen <= 0, which ROOT never writes. A corrupt length
  of 0 produced columns of N rows over an empty buffer.
• Read TBasket headers forwards, as TBasket::Streamer writes them, in one
  helper used by the container, the object reader and the decoder. Baskets
  written with TTree::kGenerateOffsetMap store a negative fNevBufSize
  followed by one fIOBits byte; reading a fixed 19-byte tail backwards from
  fKeylen misread them, so such valid files were rejected as corrupt.
• Check that the baskets of a size branch cover its fEntries before
  allocating the offsets from it.
• In auto mode fall back to ROOT on any exception from the native reader,
  not only on FormatError, and map any per-DF exception to an invalid
  timeframe. A DF number longer than 19 digits is refused as corrupt data
  instead of escaping as std::out_of_range.

Tested with a new decoder test for baskets with fIOBits, the AODNative unit
tests (34 cases), the framework root, aod-native and analysis-support tests,
and the native-vs-ROOT comparison on a production AO2D (264 DFs, 792 trees,
byte-identical).