Move 15 loose scripts from contrib/ root to contrib/scripts/

Consolidate all standalone utility scripts into contrib/scripts/:
- Perl: avg_blocktime.pl, block_time.pl, gen-zaddrs.pl, sda_checkpoints.pl,
  sdl_checkpoints.pl, hush_block_subsidy_per_halving, hush_halvings,
  hush_scanner, hush_supply, hush_supply_old
- Shell: fresh_clone_compile_and_run.sh, tidy_datadir.sh, dragonx_scanner
- Python: convert_address.py
- BAT: hush-uri.bat

Update path references in contrib/README.md, doc/OLD_WALLETS.md,
doc/relnotes/README.md, and sdl_checkpoints.pl.

doc/codebase-cleanup.md

@@ -134,4 +134,4 @@ Docs are split across `doc/`, `contrib/README.md`, and the root.
 4. Item **5** — move implementation out of headers
 5. Item **4** — split monolithic source files
 6. Item **6** — deduplicate cJSON
-7. Item **2** — Move shell scripts to `contrib/scripts/` directory
+7. Item **2** — ~~Move shell scripts to `contrib/scripts/` directory~~ Done — moved 15 loose scripts from `contrib/` root into `contrib/scripts/`