VarBinArray.DictMode.dictReadOff: U8/U16 offset ptypes fall through to 8-byte reads — IOOBE on dict-over-fsst strings #215

Found by round 2 of the Raincloud conformance triage (#205) on uci-magic-gamma-telescope (written by vortex-data==0.69.0). The class (utf8) column is a dict over an FSST values child whose codes_offsets is vortex.sequence(u8, len=3):

root: vortex.dict(utf8?, len=19020)
  codes: vortex.runend(u8, len=19020)
  values: vortex.fsst(utf8?, len=2)
    codes_offsets: vortex.sequence(u8, len=3)   # base 0, multiplier 2

VarBinArray$DictMode.dictReadOff (reader/.../VarBinArray.java:330-335) special-cases only I32/U32 as 4-byte reads and falls through to getAtIndex(LE_LONG, i) for every other offsets ptype. Here the offsets ptype is U8 while the materialized buffer holds 3×4-byte values (12 B), so reading offset index 1 attempts an 8-byte load at byte 8 of a 12-byte segment:

IndexOutOfBoundsException: Out of bound access on segment ...; new offset = 8; new length = 8
    at VarBinArray$DictMode.dictReadOff(VarBinArray.java:333)
    at VarBinArray$DictMode.getBytes(VarBinArray.java:283)

Fix directions (either/both): widen dictReadOff to handle U8/U16 (and I8/I16?) offset ptypes, and/or normalize the offsets ptype where the sequence/FSST decode path materializes offsets into a wider buffer — the ptype metadata and the actual buffer stride must agree.

Repro: hydrate the slug via Raincloud, vortex-cli export uci-magic-gamma-telescope.vortex -.