dblock / excalc-rs

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Implement general-purpose numeric integration

Ports the last remaining deferred category: trapezoid (trapez,
trapezoide), simpson, newton, boole, ordersix (ordresix), weddle (named
composite Newton-Cotes rules over a whole-number sub-interval count),
and int (gauss) (adaptive quadrature refining to a requested tolerance).

These are the only functions in v1 that take an unevaluated expression
and a bare variable as arguments rather than plain numbers, so the
evaluator special-cases their names in eval() before generic argument
evaluation, substituting the variable in a scoped copy of the context
for each sample point. The pure quadrature math lives in
src/functions/integration.rs, operating on a caller-supplied sampling
closure so it stays decoupled from the AST/Context types.

int/gauss port the original's Tegral/Gauss (Hairer's Gauss-Kronrod
quadrature with Aitken extrapolation) 'in spirit' via adaptive composite
Simpson's rule, matching the same tradeoff already made for
integration-dependent advanced/special functions, rather than
reproducing that algorithm's hardcoded coefficient tables.

Updates DESIGN.md, docs/README.md, adds docs/functions/numeric-integration.md,
folds the now-implemented note into advanced.md, adds README examples, and
updates CHANGELOG.md. This closes out every function category originally
planned for the port.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

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