zcashd sidecar: raise the reorg limit to 1000 blocks to match Zebra

[alchemydc]

Zebra raised its local rollback window to 1000 blocks in #10650 (MAX_BLOCK_REORG_HEIGHT, zebra-state v13.0.0). The zcashd fork behind the sidecar's zcashd compat mode is still on upstream's 99 (MAX_REORG_LENGTH = COINBASE_MATURITY - 1, src/main.h), so a reorg 100-1000 blocks deep shuts the sidecar node down while Zebra keeps going. Filing here because that fork has issues disabled.

Verified against valargroup/zcashd: every branch still has MAX_REORG_LENGTH = COINBASE_MATURITY - 1 and MAX_CHECKPOINTS = 100.

These have to land together — raising only the node threshold converts a clean shutdown into an assertion crash:

• src/main.h: MAX_REORG_LENGTH to 1000, decoupled from COINBASE_MATURITY (consensus-critical, must not change).

• src/wallet/wallet.h: WITNESS_CACHE_SIZE = MAX_REORG_LENGTH + 1 follows automatically to 1001. It has to track the reorg limit because CWallet::DecrementNoteWitnesses asserts nWitnessCacheSize > 0 once a note's witness deque runs dry.

• src/rust/src/wallet.rs: MAX_CHECKPOINTS 100 -> 1001. Wallet::rewind returns RewindError::InsufficientCheckpoints past that and the C++ caller wraps the FFI call in assert().

• src/rust/src/incremental_merkle_tree.rs: read_tree feeds the serialized max_checkpoints straight back into BridgeTree::from_parts, so existing wallet.dat files would stay at 100 forever. Pass MAX_CHECKPOINTS in instead, still consuming the serialized bytes so the format is unchanged.

• src/main.h: MIN_BLOCKS_TO_KEEP 288 -> MAX_REORG_LENGTH + 1, or a pruned node cannot physically reorg 1000 blocks. Pin DEFAULT_CHECKBLOCKS at 288 so startup verification does not get 3.5x slower.

• qa/rpc-tests/reorg_limit.py and qa/rpc-tests/sapling_rewind_check.py hardcode the old depths and the "roll back N blocks" messages.

Cost: witness-cache memory and wallet.dat grow roughly 10x for wallets holding shielded notes, since one witness per block per tracked note is retained. The growth is gradual — nWitnessCacheSize increases by 1 per connected block, so an upgraded wallet takes ~900 blocks to reach the new ceiling.