Promote Joypad Test Cartridge (U) from _wip to top-level project

Moves Joypad Test Cartridge (U) from _wip/ to Joypad Test Cartridge (U)/ with full ROM build:
- Adds header.s, chr.s, linker.cfg (HEADER+PRG+CHR segments)
- Updates Makefile to build and verify reconstructed.nes vs rom.nes
- Adds rom.nes copy + copilot/rom.nes symlink per repo convention
- Verified byte-identical build

Pass 1 complete: full code/data classification (~3.5% code / ~96.5% data)
Pass 2 complete: routine headers for all labels, semantic naming for key
routines (Reset, NmiHandler, VramWrite, PpuScrollLatch, VramPtrWrite,
RetAddrUpdate, etc.), zero-page aliases in locals.inc

Root OVERVIEW.md updated:
- Removes Joypad Test Cartridge from _wip skeleton table (22 -> 21 skeletons)
- Adds Joypad Test Cartridge as #14 in numbered inventory (BYTE-IDENTICAL, Pass 2)
- Updates summary count from 13 to 14 projects

Joypad Test Cartridge (U)/copilot/rom.nes

@@ -0,0 +1 @@
+../rom.nes