Tidy sort core for portability + mark canonical SSOT (fleet sortable-table primitive)

Fleet decision bs-mr4rmdgg241 (unanimous 4-PM, Elazar opener) adopts Pluto's headless sort core as fleet SSOT: src/lib/sort-utils.ts (comparePrimitiveValues + compareNullable + SortDir) + src/hooks/use-sortable-table.ts. Venus/ayudarg/mars VENDOR these two files; bin-whey-cc propagates byte-sync (same shared-tree mechanism as shared-md commons). Pluto task: (1) verify the two files have ZERO Pluto-internal coupling beyond each other (no app-specific imports) so they drop cleanly into an SCSS app; (2) add canonical-SSOT header comment marking Pluto owner + vendored-by-others + bin-whey-cc-synced; (3) confirm in-file docs cover the 3 shipped edges: tri-state asc->desc->CLEAR, deterministic id-tiebreak, nullsLast OPT-IN default=flip-with-direction. Head component sortable-table-head.tsx is shadcn-coupled, does NOT travel. Pure refactor/doc, no behavior change; do not alter sort semantics.