Extends router.sx with the full SX URL algebra — structural navigation
(.slug, .., ...), keyword set/delta (.:page.4, .:page.+1), bare-dot
shorthand, and ! special form parsing (!source, !inspect, !diff, !search,
!raw, !json). All pure SX spec, bootstrapped to both Python and JS.
Fixes: index-of -1/nil portability (_index-of-safe wrapper), variadic
(+ a b c) transpilation bug (use nested binary +). Includes 115 passing
tests covering all operations. Also: "The" strapline and essay title.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
giles
b23e81730c