Created [SPEC.md](/usr/local/src/reticulum/LXMF/SPEC.md) containing the evidence-limited `LXMessage Wire Format` section.

It documents:

- Core serialized layout and payload order
- Message ID and signature-input calculations
- Stamp exclusion behavior
- Delivery representations
- Exact source references and evidence commits
- Explicit unresolved behavior

Verification: `perl tools/verify_examples.pl` passed all checks.

Only `SPEC.md` is untracked; no other files were changed.