docs(DEN-1508): renumber deterministic Zed-to-Nix export contract

[ORESoftware]

Linear

• DEN-1508 — deterministic standalone Zed → Nix flake-bundle rendering
• Parent delivery: DEN-1418

Summary

Carries forward the complete deterministic Zed → Nix export-bundle contract from PR #25 on top of current main, while assigning it the next unique architecture number:

• docs/26-deterministic-nix-export-bundles.md contains the reviewed 282-line contract;
• README.md indexes the contract as document 26;
• architecture documents 24 and 25 remain the already-merged recursive-installer and complete-constraint-solver contracts;
• the unnumbered zed develop clean-room acceptance contract remains separate.

Semantic conflict resolution

PR #23 and PR #25 were parallel branches from the same base and both claimed document number 24. PR #24 had already identified this collision and deliberately remained unnumbered.

The resolution preserves every valid line of work instead of selecting one side:

1. merge the unnumbered clean-room acceptance contract;
2. merge recursive installs as document 24 and complete solving as document 25;
3. rebuild the Nix export contract from exact current main as document 26.

The Nix contract body is preserved from PR #25 except for the document heading and filename changing from 24 to 26. No rebase, force-push, history rewrite, wholesale side selection, or document deletion was used.

Branch state

• base: main@919104047a64e3a10bdf0f1d1a8a37f1c1cb93c4
• ahead: 2 commits
• behind: 0 commits
• changed files: 2
• runtime code changes: none

Validation gate

Keep this draft until documentation CI and agents-policy validation pass on exact head 5ab226d86f6b8c73184d5c31964f5d3408a33a2c.

Supersedes PR #25 only after this current-main replacement passes and merges.

[linear-code]

DEN-1508