SX URL algebra: relative resolution, keyword ops, ! special forms

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>

sx/sxc/home.sx

@@ -6,7 +6,7 @@
     (h1 :class "text-5xl font-bold text-stone-900 mb-4"
       (span :class "text-violet-600 font-mono" "(<sx>)"))
     (p :class "text-2xl text-stone-600 mb-4"
-      "A framework-free reactive hypermedium")
+      "The framework-free reactive hypermedium")
     (p :class "text-sm text-stone-400"
       "© Giles Bradshaw 2026")
     (p :class "text-lg text-stone-500 max-w-2xl mx-auto mb-12"