Fix invalid codegen for bool bitfields

[tannergooding]

Fixes #508.

A bool bitfield generated fully invalid C#: a bool backing field, bool & int masking, and (bool) casts, e.g.

public bool _bitfield1;
// ...
return (bool)(_bitfield1 & 0x1);                                   // invalid
_bitfield1 = (bool)((_bitfield1 & ~0x1) | (value & 0x1));          // invalid

Fix

Route bool bitfields through the same unsigned-integer path already used for the equivalently-sized integer type (a byte backing store), but keep the public accessor typed bool and convert at the get/set boundary:

• backing field is an integer (byte),
• the getter compares the masked value against zero (... != 0),
• the setter maps the incoming bool to 0/1 (value ? 1 : 0).

public byte _bitfield1;

[NativeTypeName(""bool : 1"")]
public bool a
{
    readonly get => (_bitfield1 & 0x1u) != 0;
    set => _bitfield1 = (byte)((_bitfield1 & ~0x1u) | ((value ? 1 : 0) & 0x1u));
}

Non-bool bitfields are untouched (an adjacent int c : 2 in the test emits identical output to before).

Validation

• Build: 0 warnings / 0 errors (Release).
• Generated output for a bool/int mix compiles and round-trips correctly under TreatWarningsAsErrors.
• Tests: 3710 passed, 0 failed (3708 existing byte-identical + 2 new BooleanBitfieldTest cases covering the readonly get and compatible-code accessor shapes).

Known limitation (pre-existing, out of scope)

Mixing a bool bitfield with a differently-typed bitfield of the same size in a single storage unit (e.g. bool a : 1; char b : 1;) was already emitting invalid code before this change and still is; the common all-bool case and bool mixed with different-sized fields both work.

Note

This PR description and the change were drafted with Copilot.