S4.9 — Docs, migration guide, ADR-0003 accepted

Intent

Document the 0.9.0 model and give users one migration path for all its breaking changes.

Acceptance criteria

• docs/src/concepts.md: state validity vs motion validation, resolution vs checking strategy, SpaceInformation, milestones (terms from CONTEXT.md)
• docs/src/guides/validity_checkers.md and guides/goals.md: fallible signatures, Send + Sync, error behaviour
• New guide: writing a motion validator (Rust, Python, JS) — including the "s1 assumed valid" contract and a batched/vectorised example
• docs/src/python_api.md / js_api.md: SpaceInformation, motion validators, error classes, resolution methods, max_milestones
• docs/src/contributing/implementing_planners.md / implementing_state_spaces.md: valid_segment_count, using the motion validator, setup(problem_def)
• All six example pages (docs/src/examples/*) match the updated example programs
• Migration guide 0.8 → 0.9 listing every breaking change with before/after snippets in three languages:
  • fallible is_valid / is_satisfied / distance_goal (Ok(..))
  • Send + Sync on checkers, spaces, goals (Rc/RefCell → Arc/Mutex)
  • PlanningError non-exhaustive, no PartialEq, new User variant
  • default fraction 0.05 → 0.01 and no ×0.1 (set 0.005 for the old single-space spacing)
  • SpaceInformation + ProblemDefinition + setup(pd); bindings drop setup; JS StateValidityChecker removed
  • binding exceptions; JS setter rename
  • spaces shared and frozen once used
• ADR-0003 status: accepted (repo docs/planning/adr/0003-motion-validation.md)
• Every breaking commit in the release range carries a BREAKING CHANGE: footer (check git log v0.8.0..)
• mdbook build docs clean

Source

Agreed in the senior-engineer grilling 2026-10-08 (decisions: repo docs/planning/adr/0003-motion-validation.md; glossary: CONTEXT.md). Regeneration spec: oxmpl - sprint-006. Line references are as of origin/main c558556 (2026-10-08), before E3. E3 renames things (rand Rng→RngExt, PyO3 with_gil→attach, edition 2024), so re-grep before trusting a line number.