v2.1.4: complete the v2.1.3 hygiene cleanup follow-ups

[jpurnell]

Summary

Three follow-up cleanup items deferred from v2.1.3, all bundled here as a single follow-up patch release.

Changes (1 commit)

1. Examples/ String(format:) cleanup

• MultipleLinearRegressionExample.swift: 22 calls → value.number(N) interpolation
• LinearRegressionConvenienceExample.swift: 18 calls → value.number(N) interpolation

Both example files now consistent with the rest of the codebase. Examples are not part of the package build target so these violations didn't fail CI before, but they were user-facing reference code that propagated the bad pattern.

2. generateRandomReturns seeded overload

• New: generateRandomReturns(count:mean:stdDev:using: inout some RandomNumberGenerator)
• The unseeded original is now a thin wrapper that creates a SystemRandomNumberGenerator and calls the seeded overload — same observable behavior, no breaking change for existing callers
• Fixed an edge case where Double.random(in: 0.0...1.0) returning exactly 0 would cause log(0) = -inf in the Box-Muller transform (guarded with Double.leastNormalMagnitude)

3. SeededRNG consolidation

5 test files had inlined a local struct SeededRNG using the MMIX LCG (6364136223846793005). Consolidated into a single canonical type:

• New: TestSupport.MMIXSeededRNG — value type (struct), var state: UInt64, mutating func next() -> Double returning [0, 1], mutating func nextSigned() -> Double returning [-1, 1] (the latter matches what DenseMatrixTests was doing inline)
• 5 affected files updated to import TestSupport and use MMIXSeededRNG; local struct declarations removed
• 2 of the 5 needed import TestSupport added (Descriptives Tests.swift, DenseMatrixTests.swift)
• Bit-identical sequences preserved — the MMIX constants are identical to the local declarations, so no test assertions needed re-tuning

Test plan

• All 4817 tests pass (no count change from v2.1.3)
• Zero compiler warnings
• Zero String(format: matches in Swift files (Sources/, Tests/, Examples/)
• No struct SeededRNG local declarations remain in Tests/ (only the canonical TestSupport type)
• DenseMatrixTests's [-1, 1] mapping is preserved via nextSigned()

Compatibility

• Purely additive at the public API level.
  • generateRandomReturns(count:mean:stdDev:) signature unchanged
  • New generateRandomReturns(count:mean:stdDev:using:) overload added
  • New TestSupport.MMIXSeededRNG type added
  • Existing TestSupport.SeededRNG (Numerical Recipes LCG) untouched
• No production behavior changes.

Future cleanup not in this PR

DocC .md files in Sources/BusinessMath/BusinessMath.docc/ still contain ~62 String(format:) instances in Swift code samples (12 files). These are documentation, not compiled, but they propagate the bad pattern to users who copy from the docs. Tracked as a future v2.1.5 cleanup.

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