Rewrite the Relations planning doc to current state

[JeroenDeDauw]

For #630

The Relations work map had drifted from master. Both authoring paths for
intermediate structures have shipped (mapping-side node synthesis, the
tree editor with in-place target creation), the relation representation
in RDF is recorded as decided in NativeRdfProjection.md, the cross-Source
guard moved from the closed
#1043 to
#1265, Subjects move
between pages on their own, the relations endpoint is specified and
measured in #1324, and
two links and an anchor were broken. The doc also listed shipped in-flow
creation as forward work.

Rewrite it as a work map for someone starting the implementation: what
exists, the six proposed decisions and which of them ratification gates,
the questions that are still open, and the work ordered by what it waits
on. A fifth shorter, with the PR-by-PR recital, the decided RDF question,
and the design-call reassurances gone.

Considered, omitted: a GitHub Discussion link, since no thread exists and
the epic is the venue; roadmap dates, which live in the ECHOLOT roadmap.

_{AI-authored — Claude Code, Fable 5 (max); @JeroenDeDauw asked for a from-scratch pass via the writing skill, no redirections; not yet human-reviewed; currency audited claim-by-claim against master and GitHub by a subagent, blind-judge pass applied, links and anchors checked.}