ADR-141: luria unconcretize records the given-back code under formerly: by default, and --noformerly turns it off

Context

ADR-140 added luria unconcretize and recorded no formerly:. Its reason was the collision case: the trunk holds the old number, so “an alias would claim a code that another document holds, or soon will”.

The owner reversed the default. formerly: is how the record keeps an old address resolving when the in-tree rewrite cannot reach every citation, and concretize and luria migrate both write it. A renumbering has those out-of-tree citations as much as a concretization does.

Two facts about the existing machinery decide whether the default is safe:

• A code resolves by number first. doc_refs.resolve looks the number up in documents(), and the alias map is consulted only for a spelling with no live document. So formerly: [ADR-068] on another document never takes ADR-068 from the document that holds it. test_a_live_document_outranks_the_alias pins this.
• A temporary document’s formerly: is not in the alias map. aliases.alias_map reads numbered documents only. The alias takes effect once concretize numbers the document, and that document then carries both its old number and its temporary tail.

Decision

• By default, luria unconcretize appends the old code to the document’s formerly:, with concretize’s own _record_alias.
• --noformerly records none. That is the documented choice for a collision: the old code is the trunk’s, so the branch’s document should not remember it as its own past.
• The rewrite skips formerly: blocks (doc_refs.FORMERLY_RE). Another document whose past address was the same code keeps it. Rewriting it would give that document a history it never had. This became necessary once the alias was the default, because a renumbering can meet a code some other document used to hold.

Alternatives considered

• Keep no alias by default (ADR-140). A renumbering would then break every out-of-tree citation of the old numbers. That is the failure formerly: exists to prevent, and the default should not cause it.
• A repair:-style config key instead of a flag. Whether to alias is a choice about one run of the command, which depends on whether it is a collision or a renumbering. It is not the record’s standing setting.
• Refuse the alias when another document holds the code. That would mean checking the trunk, which ADR-140 kept out of the command at the owner’s direction: the user names the documents, and the command does not judge.

Consequences

• After a renumbering, a citation of an old number that nothing else claims resolves to the document that used to hold it.
• A collision fix should pass --noformerly. Forgetting it does not misresolve, but it leaves an alias that formerly: readers, such as the site or a future check, will show as part of the branch document’s past.
• Verified by tests/test_unconcretize.py:
  • the alias is written by default and not under --noformerly;
  • concretize keeps both old spellings;
  • a live document outranks the alias;
  • another document’s formerly: entry is untouched.