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

[TMHSDigital]

Closes #369.

• Candidate pool: removed the 83 struck-through SHIPPED bullets, plus the 27 shipped entries embedded in the Sports/Nature/Household/Vehicles category lines. Those lines keep only their open subjects (skateboard; laundry basket; sci-fi supply crate; Nature: all shipped). The section intro now states the remove-on-ship rule. The shipped record stays in the CHANGELOG and git history.
• Theme table: the "Modal operators, USD, mathutils" row is now explicitly about the skills, since the USD evaluation_mode example shipped.
• "Stable (1.0)": links to a new section of proposed criteria: tested install paths for both agents, stable skill and rule names, a written Blender support policy, the measured payoff from feature: measure the payoff — agent pass rate with vs without the pack, graded by the existing headless harness #373, and content-driven plugin versions from release: content fixes under docs: never reach plugin users, while site-only commits bump the plugin version #350. These are proposals for the maintainer to adjust.
• The release-owned **Current:** line is untouched. 0 SHIPPED markers remain (grep -c SHIPPED ROADMAP.md), and check_counts.py passes.

Inspection only: this is a docs change.

🤖 Generated with Claude Code

[TMHSDigital]

CI evidence before merge: all checks pass ( 10 pass ). No Blender Smoke run (paths-ignore: docs-only).