Update to unified kth/0.78.0 Conan 2 package + modern CI

The standalone c-api/<ver>@kth/stable recipe was rolled into a single
unified `kth` package; this branch updates py-native to consume it,
modernizes the build pipeline, brings the C bindings up to date with
the C-API drift accumulated over the last ~4 years, and replaces the
deprecated CI workflow with a cibuildwheel-based one.

Build pipeline:
- conanfile.py: Conan 2 syntax (requirements()/generate()), pins
  kth/0.78.0 with currency=BCH and db=dynamic, stages headers and
  static libs from kth and ALL transitive deps into ./kth/.
- setup.py: drop dead Conan 1.x Conan.factory() path; shell out to
  `conan install` instead. BuildExtCommand discovers staged libs at
  build time and wraps them in -Wl,--start-group/--end-group so the
  linker resolves circular deps regardless of glob order. C++23,
  explicit -lstdc++. run_conan_install() also reads KTH_MARCH_ID and
  KTH_CURRENCY from env (CLI flags still win) so CI can override.
- Move .sh helpers to scripts/, drop .bat, .travis/, .appveyor.yml,
  AUTHORS, residual __init__.py.

C-API drift fixes (Python signatures changed accordingly — py-api
will need matching updates):
- block/header/transaction factory_from_data: drop `version` arg
- block/header serialized_size, header_to_data: drop `version` arg
- output_point_construct_from_hash_index: takes kth_hash_t const* now
- chain_script_construct -> chain_script_construct_from_bytes
- wallet_payment_address_encoded split into _legacy / _cashaddr
  (with token_aware bool) / _token (the BCH-only ones gated by
  KTH_CURRENCY_BCH)
- transaction_hash_sighash_type removed (no longer in C-API)
- node_get_chain -> node_chain
- node.cpp: explicitly include kth/capi/node_info.h (no longer
  pulled in transitively by kth/capi.h)

Settings struct sync:
- DatabaseSettings: drop flush_writes/file_growth_rate/
  index_start_height; add db_mode (kth_db_mode_t enum:
  0=pruned, 1=normal, 2=full_indexed).
- BlockchainSettings: drop gauss_activation_time/
  descartes_activation_time; add bch_gauss/bch_descartes/
  bch_lobachevski/bch_galois bools and the new
  leibniz_activation_time (2026) / cantor_activation_time (2027).

module.c quality pass:
- Replace 8 functions full of `int res2 = ...; res2 = ...; res2 =
  ...` (set-but-never-checked) with KTH_TRY_SETATTR macro that
  Py_DECREFs the half-built object and returns NULL on the first
  failure, propagating the Python exception correctly. The macro
  name is provisional; flagged for the cleanup pass.

Tests:
- test_config.py rewritten with shared `_assert_common_*` helpers
  (mainnet and testnet4 share ~80 asserts), named constants
  (DB_MODE_NORMAL, LEIBNIZ_ACTIVATION_TIME, CANTOR_ACTIVATION_TIME),
  loops over BIP/BCH flag families instead of 30 copy-pasted lines,
  and `>=` bounds instead of exact-equality on things that grow
  organically (checkpoint count, seed count). Covers the new
  db_mode and bch_* fields, drops the removed gauss/descartes ones.
- test_node.py: rename node_get_chain -> node_chain.

All 3 tests pass against a real Knuth v0.78.0 node startup on
mainnet (32+32 threads, march_id=ZLm9Pjh).

CI (replaces the old knuth.yml):
- The old workflow was a Frankenstein of npm/node-pre-gyp/twine/
  Docker-custom across three shells, pinned to ubuntu-20.04
  (deprecated by GitHub) and node-pre-gyp paths that don't even
  apply to a Python package. Replaced wholesale.
- New ci.yml: sdist job + cibuildwheel matrix on Linux x86_64
  (manylinux container) and macOS arm64 (macos-latest is Apple
  Silicon now). cp39-cp313, no PyPy, no musllinux, no 32-bit.
- Conan installed in CIBW_BEFORE_ALL, kth remote registered, and
  CIBW_BEFORE_BUILD pre-installs the build deps so we can run pip
  with --no-build-isolation (our setup.py shells out to `conan
  install`).
- Smoke test inside each built wheel: `import kth_native` + pytest.
- publish job scaffolded for Trusted Publishing on tag pushes,
  gated behind `if: false` until PyPI auth is wired up.
- KTH_MARCH_ID/KTH_CURRENCY pinned at workflow level so the build
  flavor lives in exactly one place.
- Concurrency group cancels superseded PR runs to save minutes.
- Windows is intentionally out of this pass — the old workflow's
  Windows path was 100% npm/node-gyp/node-pre-gyp and never built
  a Python wheel in the first place. Add `windows-latest` later as
  a follow-up once Linux/macOS is green.

Author: fpelliccioni