PR-2/v2-scaffold

[shmercer]

• Added Adaptive v2 canonical log schemas and append helpers plus a minimal v2 state constructor (R/adaptive_logs.R,
  R/adaptive_state.R).
• Updated adaptive entrypoints to return v2 state scaffolding and clarified stubs (R/adaptive_run.R), with guards
  against v2 misuse in v3 summaries (R/btl_mcmc_summaries.R, R/adaptive_schemas.R).
• Added Step 2 tests for logs, state constructor, and entrypoints (tests/testthat/test-5001-logs-constructors.R,
  tests/testthat/test-5002-state-constructor.R, tests/testthat/test-5003-entrypoints-scaffold.R).
• Regenerated docs for adaptive entrypoints; DESCRIPTION collate updated by devtools::document().