Copper 1.2.3

Fix a nightly compiler crash in replay callback construction that prevented docs.rs builds.

Published cu29 and cu29-runtime 1.2.3. cu29 requires runtime 1.2.3; cu29-derive remains at 1.2.2.

Local docs and docs deployment CI now use floating nightly. The public API and replay behavior are unchanged.

Validation: 426 release tests passed; lint including embedded/no_std checks passed; the packaged cu29 archive built docs with the published runtime on the current nightly using docs.rs cfg and target settings.