docs: fresh gap analysis against real JSS (replaces CSS-flavoured)

Replaces the GAP-ANALYSIS.md and PARITY-CHECKLIST.md that were written
against a Community-Solid-Server mental model. The corrected baseline
uses the JavaScriptSolidServer local clone as source-of-truth.

- GAP-ANALYSIS.md (~5960 words, sections A-I): scope/method, feature
  matrix, categorical summary, overbuilt features (keep/deprecate/
  feature-flag), missing features with priority + port tickets,
  semantic differences, net-new contributions, prioritised roadmap,
  ship-stance conclusion with AGPL-3.0 implications.

- PARITY-CHECKLIST.md (~4360 words, 97 rows): one row per JSS
  inventory feature. Counts: 58 present, 13 partial-parity,
  6 semantic-difference, 16 missing, 20 net-new, 3 explicitly-deferred,
  3 wontfix-in-crate, 2 shared-gap. Spec-normative parity 74% strict,
  79% half-credit.

- docs/reference/jss-feature-inventory.md: canonical JSS surface
  (source-of-truth for the gap analysis). Mirrored from VisionClaw.

Docs-only. No source changes. Follow-on after v0.3.0-alpha.3 tooling
pass.

Author: jjohare