hygiene: Phase 3 — dead-code excision (~7,100 lines)

Third phase of the code-hygiene remediation: remove provably-dead code.
Each in-file deletion was adversarially verified (a per-finding investigator
plus a skeptic that greps the tree to refute deletability); the sanity grep
confirms zero dangling references, and the tree builds and self-mines clean.

Removed wholesale:
- src/cc/dapps/ (9 files, ~6,939 lines): a git-tracked but never-built Komodo
  DEX / z-migration tool (hushdex.c, zmigrate.c, cJSON.c, ...). Nothing in
  Makefile.am/configure.ac references it.

In-file dead code:
- pow.cpp: the `#ifdef original_algo` oldRT_CST_RST function (the macro is
  never defined in source or build flags) and two `if ( 0 )` debug blocks.
- walletdb.cpp: the "orphaned staking transaction" cleanup path (deadTxns) —
  DragonX is RandomX PoW with no staking, so the guard is always false and the
  block never runs; also drop the now-unused static/extern decls.
- cc/eval.h: the ProcessCC / Eval::ImportCoin / ImportPayout / DisputePayout
  declarations that have no definition anywhere in the tree.
- rpc/crosschain.cpp: the crosschainproof stub RPC (returned {} unconditionally)
  and its registrations in rpc/server.cpp, rpc/server.h, rpc/client.cpp.
- coins.cpp: the commented-out `//TODO: delete` Sprout PushAnchor template.
- wallet.cpp: the permanently-zero KMD `interest2` term in CreateTransaction.
- rpc/net.cpp: hush_longestchain's always-zero `n` var (num > (n>>1) => num>0)
  and its `if ( 0 )` debug branch.
- hush_nSPV.h / hush_nSPV_fullnode.h: three `if ( 0 && ... )` dead debug branches.
- net.cpp, crypter.h, saplingconsolidation.cpp (dup set_error_code),
  shieldcoinbase.cpp (`donation < 0` on a uint8_t), cclib.cpp (unused
  FAUCET2SIZE): single-line dead-code fixes.

Deliberately NOT touched (verification refuted the audit's "dead on DragonX"
premise): the ~2,250-line HUSH3 checkpoint block, the miner notary/timelock
paths, and hush_gateway.h — all reachable at runtime via -ac_name / -ac_*
args (the inherited Komodo assetchain model), and hush_gateway's
hush_opretvalidate is called from ConnectBlock (consensus). Dropping those
requires a deliberate decision to remove HAC/HUSH3 mode, tracked separately.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitignore

@@ -131,7 +131,6 @@ src/cc/rogue/rogue
 src/cc/rogue/rogue.so
 
 src/cc/rogue/test.zip
-src/cc/dapps/a.out
 src/checkfile
 
 src/foo.zip
@@ -154,7 +153,6 @@ src/rogue.scr
 src/cc/rogue/confdefs.h
 src/cc/rogue/x64
 
-src/cc/dapps/a.out
 src/Makefile.in
 doc/man/Makefile.in
 Makefile.in