Add PLAN_PARAM_* constants to state_layout.cuh and replace raw
plan_params/pp slot accesses in experience_kernels.cu and
backtest_plan_kernel.cu.
Sites migrated: pp[0..5] in both files (12 sites), plan_params_ptr
indexed accesses at slots 0, 1, 4 in experience_kernels.cu (4 sites),
plan_params[w*6+4] in backtest_plan_kernel.cu (1 site). Loop variable
pp[p] in noisy-plan path left as-is (p is a runtime loop counter, not
a literal slot index).
No behavioural change; pure refactor.
Plan 1 Task 4C. Spec §3 Invariant 8.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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