Block the distillation pull request when a fix entry breaks the razor

[sameera]

As a maintainer of the concept store, I want the drain to run the validator in append-only-log mode on a fix entry's page changes before it opens the pull request, so that a fix can never leave a page whose body contradicts its own log.

Acceptance Criteria

• Given a fix entry whose staged page changes satisfy the razor, when the drain runs the validator in append-only-log mode against those pages, then the validator exits 0 and the distillation pull request opens.
• Given a fix entry whose staged page changes edit a Key Invariant or a Summary sentence, when the drain runs the validator in append-only-log mode, then the validator exits non-zero, no distillation pull request opens, and the reported message names the fix entry, names the page, states that changing what a page asserts is a design change, and tells the reader to plan it with /nxs.epic.
• Given an epic entry in the same run, when the drain runs the validator, then append-only-log mode is not applied to that entry's pages.
• Given a fix entry, when the drain runs its validation step, then the validator's existing checks still run against the changed pages in addition to append-only-log mode.

Notes

The gate rides the contract the drain already honours, that a non-zero exit from the validator blocks the pull request. No new gate semantics are introduced, which is the point: the razor cannot be talked past by a prompt.

This is the load-bearing half of a two-sided check. The advisory warning in /nxs.fix fails soft and is not a gate, because the authoring command writes no pages and cannot see what the drain will later synthesize. Putting the weight here puts it on the side that can see the page writes it constrains.

The refusal message matters as much as the exit code. A developer who hits it needs to learn that their change alters what a page asserts, which makes it a design change rather than a fix.

story_type: system · size: S