fix: parse stock amounts invariantly

[vycdev2]

Summary

• Parse explicit stock sell and transfer amounts with invariant decimal semantics instead of the host process culture.
• Preserve the existing positive-amount validation and all behavior.
• Add regression coverage for decimal input under a comma-decimal culture and invalid amounts.

Verification

• dotnet restore Morpheus.sln — passed; existing NU1903 SQLitePCLRaw.lib.e_sqlite3 vulnerability warning.
• dotnet build Morpheus.sln --no-restore --nologo — passed; 0 errors, same warning.
• dotnet test Morpheus.Tests/Morpheus.Tests.csproj --no-restore --filter FullyQualifiedName~StockInputParserTests --nologo — passed, 5/5.
• dotnet test Morpheus.sln --no-restore --nologo --logger "console;verbosity=minimal" — passed, 170/170.
• Targeted dotnet format ... --verify-no-changes for changed files — passed.
• Full repository dotnet format — blocked by the pre-existing CRLF/whitespace baseline in unrelated files; no formatting changes were applied.
• git diff --check upstream/main...HEAD — passed.

Risk

• Low: only the parsing path for explicit stock sell/transfer amounts changes; stock buy's existing Discord command binding and the all path are unchanged.

This was generated by an AI agent (vycdev2). Please verify any changes before merging or applying.