fix: three v1.9.4-acceptance findings (installer WARN, init sentinel, hash collation)

Unreleased; no pin moves except pi-extensions master 25c1265 -> 143a214.

1. entrypoint-user.sh: first-run sentinel is now config.json, which is what
   `mempalace init` writes. The old test on palace/ (created by mining, not
   init) re-fired "Initializing MemPalace" on every boot of a container that
   never mined locally: v1.9.4 acceptance measured 1 line after one boot, 2
   after a restart. Idempotent, so harmless; the comment lied. This file is a
   base-hash input, so the next tag rebuilds the base (326fb7c03949 predicted
   with the pinned sort below; f40c4b7b103d before).

2. Dockerfile.variant: `git config --system --add safe.directory` for the
   seven root-owned /opt clones (listed, not `*`). Since git 2.35.2 any git
   command in a repo owned by another user fails "dubious ownership"; as
   `developer` that made `git -C /opt/pi-atelier rev-parse` print nothing
   (one false FAIL in the v1.9.4 acceptance) and is the root cause of the
   per-boot "WARN: pi-extensions install.sh failed" (fixed at the source in
   pi-extensions 143a214; this is the belt to that suspender). Verified live
   in a v1.9.3 container: add -> rev-parse prints 25c1265, unset -> fatal
   again; /etc/gitconfig restored to its 5 lines afterwards.

3. docker-publish.yml: `find -print0 | LC_ALL=C sort -z` in base-decide.
   sort collates per locale; identical rootfs hashed to base-f40c4b7b103d
   under C/C.UTF-8 (== run 695) and base-d8df62216a81 under sv_SE/en_US.UTF-8.
   The runner exports LANG=C.UTF-8, so the hash was stable by accident. Pin is
   hash-neutral: pinned sort + HEAD inputs reproduces f40c4b7b103d exactly.
   Per-command prefix only; nobody's locale changes.

CHANGELOG: new `## Unreleased` above v1.9.4 naming pi-extensions 143a214
(check 9 rc=0), the three fixes, and the synlig 3.10.0 hub upgrade that
v1.9.4 listed as still open. One invented URL org (gwpl) caught before commit;
upstream is elpapi42, as Dockerfile.variant:151 says.

Gates: check-doc-drift rc=0, check-base-hash rc=0, lint-shell rc=0 (16 files),
hadolint 2.15.1 rc=0, YAML parses (10 jobs), bash -n rc=0.

.gitea/workflows/docker-publish.yml

@@ -108,7 +108,14 @@ jobs:
         id: compute
         run: |
           # Hash inputs that determine the base image's contents.
-          # Order is fixed via `find -print0 | sort -z` for reproducibility.
+          # Order is fixed via `find -print0 | LC_ALL=C sort -z`. The LC_ALL=C is
+          # load-bearing: sort collates per locale, and a dictionary locale
+          # (sv_SE/en_US.UTF-8) orders rootfs differently from byte order, giving
+          # a different hash for identical content (measured 2026-09-22:
+          # base-f40c4b7b103d under C/C.UTF-8 vs base-d8df62216a81 under
+          # sv_SE.UTF-8). The runner ships LANG=C.UTF-8 today, so this pin
+          # changes nothing now; it stops the hash depending on that accident.
+          # Predicting base_tag locally MUST use the same prefix on this sort.
           # Junk filters: __pycache__/*.pyc and macOS metadata are gitignored
           # locally but still picked up by `find rootfs -type f` on a clean CI
           # checkout. Exclude them defensively.
@@ -120,7 +127,7 @@ jobs:
                 ! -name '*.pyc' \
                 ! -name '.DS_Store' \
                 ! -name '._*' \
-                -print0 2>/dev/null | sort -z | xargs -0 cat 2>/dev/null
+                -print0 2>/dev/null | LC_ALL=C sort -z | xargs -0 cat 2>/dev/null
               cat entrypoint.sh entrypoint-user.sh
               # mempalace-toolkit is cloned in Dockerfile.base at a ref CI
               # resolves to a SHA; fold it in so base_tag changes when the