fix(ci): require the reused next-build artifact to come from this repository

CodeQL raised actions/artifact-poisoning/critical on the `next-build` fast path
this PR builds on (#8941). The finding is real and it sits on the path that
produces the published npm tarball.

The step picks a CI run by querying the runs API for `head_sha` and filtering on
`name == "CI" and conclusion == "success"`. That query also returns
`pull_request` runs from FORKS: they execute in this repository's context and
upload their own `next-build`, built from fork-controlled source. Measured
today, 57 runs in this repo have a `head_repository` other than the repo itself.
So the selection trusted bytes by coincidence of commit SHA — anything that made
a fork's head commit coincide with the publish commit could put attacker-built
bytes on npm.

Adds `and .head_repository.full_name == env.REPO` to the selection. Provenance
is now explicit; `head_sha` still carries tree-equality. Verified against the
live API using the expression extracted from the workflow itself — the same
single run (30518663668) is selected either way for the current tip, so the fast
path keeps working while every fork run is excluded.

Not a dismissal (hard rule #14) — the clause removes the flagged trust.

    node --import tsx/esm --test tests/unit/npm-publish-artifact-provenance.test.ts
    # 3 pass, 0 fail        (base: 2 pass, 1 fail)

.github/workflows/npm-publish.yml

@@ -159,6 +159,17 @@ jobs:
       # `head_sha` is the tree-equality guarantee: same commit, same tree.
       # Best-effort by design (retention is 1 day): every miss falls through to the build
       # step below, which is why the dynamic runner above matters as the backstop.
+      #
+      # The `head_repository.full_name == env.REPO` clause is a supply-chain guard, not a
+      # filter refinement. This artifact becomes the published npm tarball. `pull_request`
+      # runs from forks execute in THIS repository's context and upload their own
+      # `next-build` built from fork-controlled source, and the runs API returns them for a
+      # matching `head_sha` — 57 such runs exist in this repo today. Without the clause,
+      # anything that made a fork's head commit coincide with the publish commit could put
+      # attacker-built bytes on npm. Requiring the run to originate from this repository
+      # excludes every fork run while keeping the fast path intact (verified: the same
+      # single run is selected either way for the current tip).
+      # CodeQL: actions/artifact-poisoning/critical.
       - name: Reuse CI's next-build artifact (skips the heavy rebuild)
         if: steps.resolve.outputs.skip != 'true'
         continue-on-error: true
@@ -169,7 +180,11 @@ jobs:
         run: |
           set -uo pipefail
           RUN=$(gh api "repos/$REPO/actions/runs?head_sha=$HEAD_SHA&per_page=100" \
-            --jq '[.workflow_runs[] | select(.name == "CI" and .conclusion == "success")] | .[0].id // empty') || RUN=""
+            --jq '[.workflow_runs[]
+                   | select(.name == "CI"
+                            and .conclusion == "success"
+                            and .head_repository.full_name == env.REPO)]
+                  | .[0].id // empty') || RUN=""
           if [ -z "$RUN" ]; then
             echo "::notice::no successful CI run for $HEAD_SHA — falling back to a full build"
             exit 0