feat(safe-dom): add minimal SafeDOM package and runnable example

Introduces tools/safe-dom/ as a standalone ReScript workspace package
implementing the SafeDOM surface the orphaned examples/SafeDOMExample.res
already assumed:

- ProvenSelector / ProvenHTML: validated-input wrappers (boundary check is
  carried in the type so mount cannot fail on input shape).
- mount: proven-input mounting that returns mountResult variant.
- mountSafe / mountWhenReady: string-input helpers that validate at the
  boundary and dispatch via callbacks.
- mountBatch: two-phase (validate + existence check, then mutate) for
  best-effort atomic batch mounting; the trade-off (innerHTML mutations are
  not transactionally reversible if the DOM races mid-batch) is documented
  in-source.

Moves examples/SafeDOMExample.res into the package so the build verifies the
surface. Fixes three latent bugs in the example uncovered by the first build:

  1. mountWithValidation had 6 opening braces and 5 closing - outer switch
     was never closed.
  2. Console.log("label", value) used JS-style varargs; switched to
     RescriptCore's Console.log2 / Console.error2.
  3. Template literal interpolating Array.length(...) needed ->Int.toString
     (no implicit int-to-string in ReScript).

package-lock.json appears at the repo root because npm hoisted the workspace
on install - committing it locks the resolved tree across the workspace.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: hyperpolymath