Add design docs for state dependency graph, dataflow analysis, unused computation, and redundant copies

[AbdulmujibOladayo]

This PR adds design/spec docs for four related Soroban static-analysis features, grounding each in the repo's existing analyzer and rule conventions (src/analysis/dependency-graph/, src/graphs/stellar/call-graph/, packages/rules/soroban/src/).

Closes #833
Builds a state dependency graph (src/analysis/state-graph/) modeling Soroban env.storage() reads, writes, and the computations connecting them, so later rules can reason about non-local dependencies instead of scanning line-by-line.

Closes #834
Specifies a dataflow analyzer (src/analysis/dataflow/) that tracks variable definitions, uses, function arguments, and return values across basic control-flow paths, feeding both #835 and #836.

Closes #835
Defines SorobanUnusedComputationRule (packages/rules/soroban/src/unused-computation-check.ts), which flags computed values that are bound but never subsequently read, returned, or passed as an argument, using the #834 dataflow output.

Closes #836
Defines SorobanUnnecessaryCopyRule (packages/rules/soroban/src/unnecessary-copy-check.ts), which flags redundant let x = y.clone(); copy chains while explicitly excluding function-parameter-sourced values to avoid ownership-related false positives.

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