docs(http): document infinite-scroll/visible observer root semantics (scrollable ancestor vs viewport)

[ErickXavier]

Follow-up from no-js-dev/nojs#302 (shipped in nojs PR #310): all three IntersectionObservers of the get/pagination directives now use the nearest scrollable ancestor (overflow-y auto|scroll) as observer root, falling back to the viewport at document level; rootMargin/threshold are container-relative; a sentinel visible inside a not-yet-overflowing container still triggers a load (fill-until-overflow semantics). references/directives.md (http/pagination section) and any SKILL.md mention of get-trigger="scroll"/"visible" should document this behavior so generated code sets expectations correctly.

[ErickXavier]

Fixed in PR #73 (merged, 4c1fdde). references/directives/http.md now documents the observer root resolution (nearest overflow-y auto|scroll ancestor, documentElement → null/viewport), container-relative get-threshold, and fill-until-overflow semantics; the get-trigger table rows for visible/scroll corrected from "viewport" to scroll container; stale viewport wording in references/patterns/data-fetching.md (3 sites) updated with a cross-reference; SKILL.md pagination summary got a one-line note. All claims verified against Core src/directives/http.js with line refs. Ships alongside Core v1.20.1. Remaining known doc mismatch tracked separately in #74 (cursor extraction).