docs: add Python >= 3.12 modernization task specification and plan

[egparedes]

Working document for a multi-PR campaign that brings the codebase onto a single
Python >= 3.12 dialect and upgrades the QA that keeps it there. No code changes
here — this is the specification and plan the rest of the stack implements.

It is deliberately evidence-first: every number was measured against this commit
rather than estimated, so the scope argument can be checked rather than taken on
trust.

What it contains

• Baseline (§2): 1905 UP findings in src/, plus an inventory of the
  stdlib shims, version-gated code, and the two eve structures the campaign has
  to deal with first.
• Ruff proposal (§3): bump to 0.16.4, and a per-ruleset recommendation with
  measured hit counts — what to enable, and what to reject with the reason
  recorded, so the question is not reopened every six months. EM is rejected
  because it contradicts the error-message rules in CODING_GUIDELINES.md; TC
  because it would move imports behind TYPE_CHECKING, which eve resolves at
  runtime.
• Risk register (§4): four of the seven entries share one failure mode — they
  do not raise. That is what makes this different from an ordinary lint sweep.
• Plan (§5): six PRs, split by how a reviewer checks each one rather than by
  topic, with the reasoning for what is merged and what is kept separate.
• Documentation work (§6) and audit findings (§7), including the gaps that
  are deliberately not fixed and why.

Notable findings that came out of writing it

• The tests/** ruff exclusion cannot simply be deleted: its 2241 findings are
  dominated by pytest and DSL idioms (413 F811 from fixtures, 213 F841 from
  traced @field_operator bodies), so it needs per-file-ignores, not removal.
• CHANGELOG.md is only touched by release commits, so entries come from PR
  titles at release time — the plan reflects that.
• Two stale cross-references in the existing docs (CODING_GUIDELINES.md points
  at a directory that does not exist; onboarding.md at a moved file), and a
  contradiction between AGENTS.md and CODING_GUIDELINES.md on Sphinx roles in
  docstrings that needs a decision either way.

Deleting this file, or folding the durable parts into CODING_GUIDELINES.md and
an ADR, is part of the last PR in the stack.

[egparedes]

Closing: the implementation plan is a local working document and should not land upstream. The two implementation PRs (#2841, #2842) no longer carry it — #2841 now targets main directly.