Harden source generation and diagnostics

[RejectKid]

What changed

• isolate source generation into one deterministic file per bit object so malformed declarations cannot suppress unrelated output
• classify backing fields from Roslyn symbols, including nullable byte[], and emit escaped, fully qualified identifiers and framework/library references
• make raw byte* behavior explicit: checked mode reports BITSKIT009, while unsafe mode generates direct pointer access
• validate invalid options, unsupported backing types, names, collisions, widths, fixed layouts, and modifier combinations with BITSKIT007–BITSKIT014
• tolerate malformed attributes and support compatible attributes derived from BitFieldAttribute
• document the generator guarantees, diagnostics, and raw-pointer contract

Why

The generator previously depended on display strings and generated all bit objects as one output. Legal nullable/keyword/shadowing cases could produce invalid C#, a malformed declaration could affect unrelated objects, and raw pointers were wrapped as zero-length checked spans. These changes make generated output deterministic, symbol-correct, isolated, and diagnosable.

Validation

• dotnet build BitsKit.sln -c Release --no-restore
• dotnet test BitsKit.sln -c Release --no-build (4,616 tests on both .NET 8 and .NET 10)
• dotnet pack BitsKit/BitsKit.csproj -c Release --no-build
• benchmark discovery on .NET 10 via --list flat

Regression coverage includes nullable arrays, unsafe raw pointers, keyword identifiers, consumer types shadowing framework names, same-named objects in separate namespaces, invalid-object and invalid-field isolation, derived attributes, and all new diagnostics.