Promote CIRCUS from _wip to top-level project

Moves CIRCUS from _wip/CIRCUS to CIRCUS/ 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

Updates root OVERVIEW.md:
- Removes CIRCUS from _wip skeleton table (25 -> 23 skeletons)
- Adds CIRCUS as #12 in numbered inventory (BYTE-IDENTICAL, Pass 2/3)
- Updates summary count from 11 to 12 projects

CIRCUS/copilot/header.s

@@ -0,0 +1,2 @@
+.segment "HEADER"
+    .incbin "header.bin"