Stage 1.11: Conventions machinery — deprecation helper, exception tree (implementation)

[jepegit]

Part of Stage 1 — behavior-preserving construction (see the Stage-1 tracking issue). Stage 0: #439. Plan documents live in the shared workspace: cellpy-workspace/architecture-plan/ (the architecture-plan repo; formerly architecture-plan/) (alongside the cellpy and cellpy-core repos). Everything in Stage 1 lands on master with the full suite green; nothing flips user-visible behavior.

Goal

Implement the conventions plan's shared machinery (specified in Stage 0 #437; this is
the build issue if not already closed there): cellpy._deprecation.warn_once,
the self-registering DEPRECATIONS.md table, cellpy/exceptions.py re-rooted on
cellpycore.exceptions.CellpyError with CorruptCellpyFile / ConfigurationError /
UnitsError / LoaderError, and the contributing-docs checklist line.

Why

Stage 1.4 (typed file errors), 1.8 (prms shim) and every later shim import from this
module; it must merge before the first of them. Kept as its own small PR so the
consumers' diffs stay reviewable.

Links

• architecture-plan/cellpy2-conventions-plan.md
• Stage-0 spec issue: Stage 0.10: Conventions bootstrap — deprecation helper, exception tree, DEPRECATIONS registry #437 (close both together if Stage 0.10: Conventions bootstrap — deprecation helper, exception tree, DEPRECATIONS registry #437 already delivered).

Acceptance

• warn-once semantics tested; registry renders DEPRECATIONS.md; one existing
  deprecation (helpers.make_new_cell) converted as the demonstrator.

[jepegit]

Heads-up for pickers: the actual implementation work here overlaps #437 (Stage 0.10), which now carries the yolo label and should be picked first. This issue stays open only to track the Stage-1 dependency edges (#449 and #453 import the conventions machinery) — close it together with #437, do not implement twice.