docs(adr): compress to 1 record in the domain-modeling format

[lesnik512]

Rewrites docs/adr/ from 1 record (301 words) to 1 (151 words), a single paragraph, in the shape
ADR-FORMAT.md asks for. The record survives the admission test, so nothing is dropped, merged or
renumbered. Same treatment as modern-python/compose2pod#102.

Evaluation

Tested against the admission test: hard to reverse, surprising without context, the result of a real
trade-off.

Old	Outcome	New	Reason
0001 expose installs into modules only; no FixtureSet install seam	kept, compressed	0001	A standing "no" on a public-surface seam: a reader who sees the bare setattr loop will propose extracting it, which is exactly the surprise an ADR exists to pre-empt, and the two-adapters rule is the real trade-off.

The rewrite drops the **Decision:** line, the **Revisit trigger:** block and the
"the test suite collapsed accordingly" narrative. The revisit trigger folds into the closing clause,
because the arrival of a second install target is the actual boundary of the decision, not an
afterthought.

Citations

None moved. The only citation in the repo is the ADR-0001 example in docs/agents/domain.md, and
both its number and its parenthetical title are still accurate. The repo-wide sweep for
adr/NNNN… / ADR-NNNN finds no old or dropped number.

Stale claims corrected

• The old record credited _collect_fixtures with "discovery, the skip-non-Provider rule,
  cross-group collision detection". Only the collision check is its own: discovery and the
  skip-non-Provider filter live in modern_di.Group.get_named_providers(). The rewrite says what
  _collect_fixtures actually does, which also includes the TypeError on an empty call that the
  old record never mentioned.
• "the public surface stays at two symbols" is now named concretely, expose and
  modern_di_fixture, matching modern_di_pytest/__init__.__all__.

Everything else verified against modern_di_pytest/factory.py: _collect_fixtures is private and
pure, the install targets are the inspect.stack() caller module and module=, both
types.ModuleType, and the install step is a setattr loop over the returned mapping.

Gates

• pytest -q tests/test_adr_citations.py: 7 passed.
• pytest -q (whole suite, 25 tests): passed. No directory needs external services.
• eof-fixer . --check: clean.
• lychee --offline over the touched ADR plus CONTEXT.md and AGENTS.md: 2 OK, 0 errors.
• ruff format --check / ruff check --no-fix: skipped, no Python touched.