[Dart] Fix struct builder alignment

[KirtiRamchandani]

Fixes #9099.

Problem
Dart generated fixed-struct builders write fields one at a time. When an inline struct with a larger alignment follows an unaligned table field, the builder can align only the first scalar write instead of the whole struct.

Root cause
The Dart runtime and generator did not have a struct-preparation step that accounts for the struct's minimum alignment and total byte size before writing its fields.

Solution

• Add Builder.prepStruct(alignment, size) to pad for a whole inline struct before individual field writes.
• Emit prepStruct(minalign, bytesize) for generated Dart fixed-struct builders and object builders.
• Add a regression case covering a ushort table field followed by a Header struct containing a ulong.

Tests run

• cmake -S . -B build -G Ninja -DFLATBUFFERS_BUILD_TESTS=OFF
• cmake --build build --target flatc
• build/flatc.exe --dart --gen-object-api -o dart/test dart/test/bool_structs.fbs
• git diff --check
• dart test from dart/