docs: pass MITC4 numerical review

docs/io-definitions/linear-static-mitc4-shell-io.md

@@ -8,13 +8,13 @@
 - source_formulation: `docs/formulations/mitc4-shell-formulation.md`
 - source_numerical_review: `docs/numerical-reviews/linear-static-mitc4-shell-review.md`
 - source_commits: requirements/research/formulation policy revision `73df844`
-- status: `approved-ready-for-numerical-review`
+- status: `approved-for-implementation-planning`
 - owner_agent: `io-definition-agent`
 - date: `2026-08-12`
 - authoritative_output: `results.h5`
 - hdf5_schema_version: `0`
 - reference_cases: read-only S4 at `reference/shell/`; read-only S4R at `reference/shellR/`
-- implementation_planning_authorized: `false`
+- implementation_planning_authorized: `true`
 
 This document defines the approved semantic input and output contract for the
 linear-static MITC4 shell feature. It does not define parser, model, HDF5-writer, or
@@ -650,7 +650,8 @@ administration and reference-portfolio expansion are removed scope.
 
 ### 11.4 Implementation Planning Agent
 
-- Do not start while `implementation_planning_authorized=false`.
+- Planning is authorized by the numerical review; implementation and Harness execution
+  remain separately unauthorized until explicitly requested.
 - After approval, convert every supported/unsupported keyword row, source identity,
   geometry/director error, drilling-load projection, exact HDF5 path/shape/order,
   atomic failure, source-row normalization, and U-versus-UR decision into