Stage 0.10: Conventions bootstrap — deprecation helper, exception tree, DEPRECATIONS registry

[jepegit]

Part of Stage 0 — foundations for cellpy 2 (see the tracking issue). 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).

Goal

Land the shared conventions machinery every later shim imports:

• cellpy._deprecation.warn_once(name, replacement, removal="2.1") (once-per-site registry,
  stacklevel correct),
• self-registering deprecation table rendered to DEPRECATIONS.md,
• exception-tree skeleton: cellpy/exceptions.py re-exports cellpycore.exceptions.CellpyError
  as the root; CorruptCellpyFile, ConfigurationError, UnitsError, LoaderError stubs,
• test-convention checklist line ("warnings emitted once, registered in DEPRECATIONS.md")
  added to the contributing docs.

Logging changes (no import-time config) are not in this issue — they ride with the
config plan's import-time-I/O removal.

Why

The config, header-migration, unit and utils plans all introduce shims with warnings. If the
helper does not exist before the first shim lands, we get four ad-hoc warning styles and a
deprecation table that is stale on arrival (conventions plan §5).

Links

• architecture-plan/cellpy2-conventions-plan.md (§1, §3, §5)

Acceptance

• Helper + registry in tree with tests (warn exactly once; registry renders).
• First consumer wired (can be an existing deprecation, e.g. helpers.make_new_cell).

[jepegit]

Note: #456 (Stage 1.11) describes the same deliverable from the Stage-1 side. Implement here; close both together.