docs(roadmap): keep only open candidates and define what 1.0 means (#425)

The candidate pool had 140 bullets, 83 of them struck-through SHIPPED
entries with full measurement detail (plus 27 more inside the category
lines), although CLAUDE.md says to remove a subject when it ships. A
reader could not see what is next. Shipped entries are gone (their
record is in the CHANGELOG and git history); the category lines keep
only their open subjects.

The theme table's "Modal operators, USD, mathutils" row now says it is
about skills (the USD example shipped, the skill has not), and "Stable"
links to a new section with proposed 1.0 criteria: tested install paths,
stable skill/rule names, a written Blender support policy, a measured
payoff (#373) and content-driven plugin versions (#350).

Closes #369

Signed-off-by: TMHSDigital <154358121+TMHSDigital@users.noreply.github.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Author: TMHSDigital