refactor: host migration state, plan entries and SQL-test planning in Rust

[kvlonge]

Why

Continue Rust-hosted planning while preserving migration state formats, SQL artifacts, adapter overrides and plan output.

Changes

• Native scenario SQL and artifact planning, SQL-test plan assembly, cursor windows and fixture validation, and fixture read resolution.
• Native audit and model plan-entry assembly, including microbatch limits.
• Native migration presentation, discovery, old-name planning, event projections and row codecs, and migration state reads, writes and inspection. State stays append-only with unchanged formats; adapter calls remain Python callbacks.
• Planner differential compares dates derived from the run date as offsets, so baselines no longer drift by calendar day.
• Delete replaced Python; movable Python decreases from 33,506 to 27,784 lines.

Verification

• 11,425 unit/integration tests passed; make check-ci passed with all optional dependencies available.
• Compiler differential: 371 projects with discovery/render coverage and unchanged goldens.
• Planner differential: all 11 projects across 18 stateful steps matched a baseline recorded from main.
• Cold and edited compile outputs and exit codes match the reference on synthetic and private compile-only projects.
• Worker before/after captures cover migration SQL, row decoding, retries and errors, fixture and SQL-test diagnostics, scenario plans and plan entries.
• Independent combined review: no validated in-scope findings.