Port src/impl/btc/coin/header_chain.hpp to BCH: same store/sync/persistence
and SHA256d PoW, with the one BCH divergence being difficulty. Replaces
Bitcoin's 2016-block 2-week retarget with ASERT (aserti3-2d) by wiring
validate_difficulty()/get_next_work_required() to asert.hpp (slice 18a).

- BCHChainParams carries the ASERTParams anchor (asert.hpp) instead of a
  (timespan, spacing, interval) tuple; pow_limit/allow_min_difficulty mirror
  the anchor. mainnet/testnet3 genesis shared with Bitcoin (pre-fork); BCH
  testnet4 genesis flagged TODO-verify against VM300 bchn-bch chainparams.
- validate_difficulty drops the ancestor-walk lookback (ASERT is anchor-
  absolute): trusts pre-anchor history and synthetic checkpoint seeds, else
  asserts header.nBits == get_next_work_required_asert(tip,...).
- No p2pool-merged-v36 surface: ASERT governs header accept only, not the
  PoW hash or share format.

Single-coin, src-only (no impl_bch CMake registration). Syntax-clean
-std=c++20. Push held pending PR #106 ratification.

Standalone-parent commitment layout (BTC minus AuxPoW segment):
[BIP34 height][/c2pool/][state_root 32B][TheMetadata]. BIP34 enforced
per BCHN ContextualCheckBlock; no witness commitment (SegWit struck);
CTOR handled separately by template tx sort; CashTokens transparent.
Source-only header; shared by M4 GBT builder and the share validator.

…e 1)

Flesh out bch::coin::TemplateBuilder (was an M4 TODO stub) into a working
GBT assembler from HeaderChain + Mempool, mirroring the BTC builder with the
BCH consensus divergences:

  * SHA256d merkle root + 50-BCH/210k-halving subsidy (== BTC curve).
  * Next-block bits from the ASERT (aserti3-2d) DAA, not a 2016-block
    retarget; falls back to tip bits / pow_limit below the ASERT anchor.
  * CTOR (CHIP-2018-11): block body re-sorted to ascending txid after fee
    selection, coinbase excluded.
  * No SegWit: single canonical tx serialization, hash == txid, empty
    segwit-free rules array. CashTokens carried transparently in tx bytes.
  * EmbeddedCoinNode wired to build_template() for getwork().

Coinbase tx (and its s19 commitment) is still assembled downstream, as on
BTC; binding ../coinbase_commitment.hpp into the coinbase builder is the
next M4 slice. ABLA dynamic block-size and the rules array are pinned to
pre-ABLA 32 MB EB / empty here pending verification vs VM300 bchn-bch GBT
and the p2pool-merged-v36 python reference.

Source-only, -fsyntax-only clean. No impl_bch CMake registration (does not
race ci-steward); bch lane stays skip-green.

…sourcing

Add abla.hpp -- a 1:1 fixed-point port of BCHN consensus/abla.{h,cpp}
(pinned at v29.0.0): Config (MakeDefault/SetMax) + State (NextBlockState,
GetBlockSizeLimit). Mainnet grows; testnets fixed at 32 MB.

Route the template builder block-size byte budget through abla at the
activation/floor limit (32 MB) instead of a static constant. The floor is a
safe LOCAL build cap -- ABLA only ever raises it, so a template built to the
floor can never exceed the live consensus limit. Per-tip ABLA state replay
through the header chain (dynamic limit) is the next size-slice.

No p2pool-merged-v36 surface: ABLA governs only the block-size limit, not
PoW hash, share format, coinbase commitment, or PPLNS math.

Roundtrip exec: floor==32MB both nets; mainnet->41.3MB after 5000 full
blocks; testnet fixed. abla.hpp + template_builder.hpp -fsyntax-only OK.

Add abla::replay() -- replay ABLA State forward from a known-good anchor
over a contiguous run of full-block sizes (1:1 BCHN State::NextBlockState
loop). Wire TemplateBuilder::build_template to consume an optional caller-
supplied per-tip ABLA State, using its GetBlockSizeLimit() as the dynamic
block-size budget; absent it, fall back to the activation/floor limit
(unchanged, always-safe LOCAL build cap).

The per-block-size feed that drives replay is a full-block/embedded-daemon
concern (M5+): the headers-only SPV header_chain structurally does not carry
block sizes, so the dynamic limit is re-homed there rather than forced into
the header chain. Build budget only -- zero p2pool-merged-v36 surface
(no PoW hash, share format, coinbase commitment, or PPLNS touched).

Validated: mainnet replay 5000x32MB grows 32->39.48MB (bounded by
MAX_CONSENSUS_BLOCK_SIZE); testnet fixedSize stays 32MB; empty replay = anchor.