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feat(ucalc): LNS-specific steps visualization (#679)

* feat(ucalc): LNS-specific steps visualization (#656)

Add log-domain step-by-step arithmetic decomposition showing:

Multiplication (trivial in LNS):
  1. Decompose: sign, log2(|value|)
  2. XOR signs
  3. Add log-values: log2(|A*B|) = log2(|A|) + log2(|B|)
     "just one fixed-point addition -- no multiplier hardware needed"

Addition (the hard case):
  1. Decompose operands
  2. Compute log-difference: d = log2(|B|) - log2(|A|)
  3. Gaussian logarithm: log2(1 + 2^d)
     "requires table lookup or CORDIC -- this is the COST of LNS addition"
  4. Add to larger operand's log
  5. Result

Division (also trivial): subtract log-values.

Students see why LNS makes multiplication and division free but pays
for addition with the Gaussian logarithm function.

Resolves #656

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix(ucalc): resolve CodeRabbit review items for LNS steps

Three fixes:

1. Add #include <limits> for std::numeric_limits::infinity().

2. Handle 0+0 fast path: returns clean "both operands zero -> 0"
   instead of NaN from -inf - -inf in the Gaussian log math.

3. Gaussian logarithm annotations now note "current library: uses
   double conversion fallback" alongside the theoretical cost model,
   since lns::operator+= delegates to double conversion.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix(ucalc): remove redundant std::ldexp(1.0, 0) in LNS multiply

std::ldexp(1.0, 0) == 1.0, making the multiplication a no-op.
Simplified to direct std::pow(2.0, result_log).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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