Add unparse functions for DLL and ELL

[tudborg]

This also adds ability to swap out the identification number when rekeying ELL.

This also changes wmbus parsing length field checks such that it doesn't mutate the context length option.

Skipped mbus unparsing for now, until I have a test case for it.

[Copilot]

Pull request overview

This PR extends Exmbus’ ability to reconstruct (unparse) WM-Bus frames by adding unparse/encode functions for ELL and DLL layers, and refactors Exmbus.Util.rekey_ell/3 to re-encrypt and rebuild frames (including optionally swapping the identification number during rekeying). It also adjusts WM-Bus length handling to avoid mutating the :length option in the parse context.

Changes:

• Refactor Exmbus.Util.rekey_ell to take input/output option sets and rebuild frames via new ELL.unparse/1 and DLL.unparse/1 flows.
• Add encode/unparse support for ELL structures and WM-Bus DLL (plus identity encoding helpers).
• Update/utilize tests for the new rekey_ell/3 behavior and identification number swapping.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
test/util_test.exs	Updates tests to exercise rekey_ell/3, including identification swapping and optional length output.
lib/exmbus/util.ex	Refactors rekey_ell to parse → decrypt → mutate → encrypt → unparse, enabling identification swap and length output changes.
lib/exmbus/parser/identity.ex	Adds encode/1 to serialize identity blocks used by ELL receiver addressing.
lib/exmbus/parser/ell/unencrypted_with_receiver.ex	Adds encode/1 for the “unencrypted with receiver” ELL variant.
lib/exmbus/parser/ell.ex	Adds unparse/1 for ELL variants to prepend ELL headers back onto ctx.bin.
lib/exmbus/parser/dll/wmbus.ex	Refactors length handling, adds unparse/1, and introduces C-field encoding for WM-Bus DLL reconstruction.
lib/exmbus/parser/dll/mbus.ex	Adds an unparse/1 stub for M-Bus (currently raises).
lib/exmbus/parser/dll.ex	Adds unparse/1 routing to DLL-specific unparse implementations.

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