Add portfolio allocation planning with Deep snapshots

[AIVensk]

Summary

Closes #138.

• Add a decimal portfolio allocation planner with validated percentage targets and conserved total value, including holdings absent from the target allocation.
• Store immutable target snapshots as typed Deep links connected to the portfolio and asset identifiers. Targets can be read back through a fresh wrapper around the same FinancialStorage.
• Add an explicit --balance-portfolio <json> CLI mode, documented example, and an xUnit project in both solutions.

Verification

• dotnet test Bot.sln --no-restore -c Release -f net8: 24 passed using .NET SDK 8.0.425.
• Tests cover the issue's 25/25/50 example, validation, rounding/value conservation, large totals, tiny percentages, Unicode/culture-independent Deep round trips, immutable snapshots and command errors.
• Ran the actual offline example: Gold +15,000, USD -5,000, stocks -10,000; net value change zero.
• git diff --check: pass.

Existing restore/build warnings (including System.CommandLine.Parser framework compatibility and obsolete upstream broker properties) remain unchanged.

Scope

The algorithm returns value changes in a common valuation currency. It does not place orders or infer prices, FX rates, fees or share quantities. Deep snapshots use the existing heap-backed FinancialStorage and live for the lifetime of that store; this does not claim disk persistence. All verification used synthetic data.

Submitting for the 1,000 RUB reward advertised in #138, subject to maintainer acceptance. Implementation and tests were prepared with Codex assistance.