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/linker.cfg

@@ -0,0 +1,10 @@
+MEMORY {
+  HEADER: start = $0000, size = $0010, fill = yes;
+  PRG:    start = $C000, size = $4000, file = %O, fill = yes;
+  CHR:    start = $0000, size = $2000, file = %O, fill = yes;
+}
+SEGMENTS {
+  HEADER: load = HEADER, type = ro;
+  PRG:    load = PRG,    type = ro;
+  CHR:    load = CHR,    type = ro;
+}