Release 2.0.0

Stable release, covering the whole arc from 1.1.8 through 2.0.0-rc.4. Highlights below; full details are in
the beta/rc entries beneath this one. The only change since rc.4 itself is CI-only: release.yml gates the
GitHub release job behind a manual-approval environment and no longer deletes a pre-existing release for the
same tag before publishing.

Changed

• BREAKING: bclibc no longer throws. Every fallible function returns BCLIBC_Result<T> wrapping one
  tagged BCLIBC_Error, replacing the old exception classes and the BCLIBC_BaseResult/BCLIBC_EngineResult
  split. The flat C ABI, which already returned error codes, is unaffected at that boundary.
• BREAKING: the RK45 tolerance/stats API is now per-instance. The thread-local
  BCLIBC_cashKarpSetRelativeTolerance-style functions are replaced by BCLIBC_CashKarpIntegrator/
  BCLIBC_DormandPrinceIntegrator/BCLIBC_TsitourasIntegrator classes, each owning its own tolerances and
  accepted/rejected step counts (safe to share across threads via std::ref).
• Event/row interpolation is interval-based, not per-raw-point, and a scheduled sample no longer silently
  merges with a physical event at a nearby timestamp — both fix real accuracy/row-count bugs that adaptive
  stepping exposed.
• tiny_bclibc migrated its adaptive core from Cash-Karp to Tsitouras 5(4), matching bclibc's own default.
• The bare WebAssembly build (BCLIBC_WASM_BARE=ON) is one flavour for both toolchains (wasi-sdk, zig) now
  that the core never throws.

Added

• Dormand-Prince 5(4) and Tsitouras 5(4) adaptive integrators, alongside the existing Cash-Karp.
• Bare (import-nothing) WebAssembly builds for both bclibc and tiny_bclibc, verified bit-identical to
  native.
• BCLIBCFFI_interpolate_trajectory_data, BCLIBCFFI_hermite, BCLIBCFFI_interpolate_3pt/_2pt in the flat
  C ABI.
• CI actually runs the C++ test suite and the tiny_bclibc identity test on every PR, plus a dedicated
  "no exceptions or RTTI in the core" guard.

Removed

• The Emscripten-based wasm build (build_wasm.sh) and its automated emsdk-bump workflow, superseded by the
  bare wasm build.