fix(build): sync VERSION into every .el header, not just the resource

[BuddhiLW]

Closes CLJEL-SYNC-VERSION (20260905161109-0e698b23), CLJEL-MELPA propagate-VERSION
(20260713171046-5d818865, the same concern filed twice) and CLJEL-TEST-TMPDIR
(20260905173027-253109a0).

Stacked on #27, which is stacked on #26. Bases retarget as those merge.

The drift

sync-version propagated /VERSION to resources/clojure-elisp/VERSION and
nothing else, while six locations restate the version. Measured on this branch
before the fix:

VERSION                     0.7.2
clojure-elisp-runtime.el    0.7.2   (stamped by compile-runtime at regen)
clojure-elisp-mode.el       0.7.1   <- drifted
cider-clojure-elisp.el      0.7.1   <- drifted

Historically those two sat at 0.5.0 while VERSION read 0.6.1 — two releases
behind, invisible because no gate looked at them, hand-fixed twice. MELPA
Stable reads that header, so drift ships the wrong version.

The fix

sync-version! stamps all three .el packages, and throws when a header is
absent rather than writing nothing. That matters: the sync writes by regex, so
a renamed or restructured file would otherwise drop out of it silently — and
silence is exactly what let this last two releases.

Stamping the runtime header here too means a bump no longer needs a regen to
reach the shipped file. (I hit that trap myself in #26: regenerated before
bumping, and shipped a runtime naming the previous release.)

Two tests, both non-vacuous:

• every .el header equals /VERSION — verified to fail when one is edited back
• no .el in resources/clojure-elisp is missing from the list, so a new
  package cannot quietly keep whatever header it was born with

Also: a flake that kept masking runs

cross-file-warning-test passed (.getParent f1) — the shared system temp
directory — to compile-project, so it compiled every .cljel any other
process happened to leave in /tmp. It errored twice during this session and
passed on re-run, which is the worst kind of red: unreproducible locally,
intermittent in CI.

Proven rather than assumed. With one unparseable .cljel dropped in /tmp:

old code   ERROR ... Reader error at line 536: Unsupported escape character
new code   1 tests, 6 assertions, 0 failures

609 Clojure tests, 3063 assertions, 0 failures. 28 ERT tests, 0 unexpected.