Replace `LMDB.mdb_unpack(::Type{T}, ::Ref{MDB_val})` — the package's
typed-read customization point — with a plain `IO` view over the
LMDB-owned `MDB_val`. Users now plug in custom decoders by defining
the idiomatic `Base.read(io::IO, ::Type{MyType})`, which works against
any byte stream and is the convention every other Julia decoder uses.

  struct PrefixedBlob end
  function Base.read(io::IO, ::Type{PrefixedBlob})
      skip(io, 8)
      return read(io, String)
  end

  LMDB.tryget(txn, dbi, key, PrefixedBlob)

`MDBValueIO <: IO` exposes `position` / `seek` / `seekstart` / `seekend`
/ `skip` / `bytesavailable` / `eof` and overrides `unsafe_read` so
Base's primitive `read(io, T)` for `Int8…Float64`/`Bool`/`Char`/`Ptr`
inherits zero-allocation. The default `read(io, String)` and
`read(io, Vector{E})` consume the rest of the buffer and copy out.

Internal call sites in `cur.jl` / `dbi.jl` / `dicts.jl` switch from
`mdb_unpack(T, ref)` to `read(MDBValueIO(ref[]), T)`. The package
exposes `LMDB.MDBValueIO` for users who override `Base.read`.

`mdb_unpack` is dropped — breaking change for the (rare) caller who
defined their own `mdb_unpack` method.

Preserves PR #52's GC-safety fix: `common.jl` keeps the
deferred-pointer-extraction pattern from #52 (the upstream commit
this is based on pre-dates that fix). `MDBValue(::String)` and the
eager-extraction variants stay deleted; `cconvert(::Ptr{MDB_val}, x)`
returns an `MDBArg` carrier with an uninitialized box, and
`unsafe_convert(::Ptr{MDB_val}, ::MDBArg{D})` does the extraction
inside ccall's preserve window.

Tested locally on Julia 1.10, 1.11, 1.12, 1.13.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>