✅ test: unskip the doctests

[nstarman]

Closes #17.

Every doctest now runs on both diffrax 0.6 and 0.7. Previously six examples
carried .. skip: next if(DIFFRAX_LT_070, ...), and README.md was dropped
wholesale when diffrax < 0.7 — so CI's check_oldest job verified none of them.

Scoping it empirically

Rather than assume all six guards were load-bearing, I removed every skip and
ran the suite against CI's own oldest resolution
(uv run --group test --resolution lowest → diffrax 0.6.0, equinox 0.11.5,
jax 0.4.35). Result: 8 failures, narrower than the guards:

• One of the six skip directives (diffeq.py, the Solution repr) guarded an
  example that passes on 0.6 anyway — its trailing ... already absorbed the
  difference.
• README.md was being dropped entirely to cover 3 failures out of ~52
  examples.

The fix

All 8 failures were reprs of diffrax-owned objects. They now elide what belongs
to diffrax and stay exact on what belongs to diffraxtra:

Varies across 0.6 → 0.7	Now written as
default-valued fields printed vs hidden	DiffEqSolver(...solver=Dopri5(...)...)
weak_i64[1] vs i64[1]	ts_size=...i64[1]
<class 'diffrax...._Dopri5Interpolation'> vs the bare dotted path	interpolation_cls=..._Dopri5Interpolation...

ELLIPSIS is already in optionflags, so this needs no new configuration.

With no skips left, DIFFRAX_LT_070, setup_namespace, ReSTSkipParser and
pytest_collection_modifyitems have nothing to do — conftest.py drops them
and the diffrax and packaging imports they required, going from 62 to 39
lines.

Verification

• diffrax 0.6.0: 158 passed
• diffrax 0.7.0: 158 passed
• nox -s mypy clean, pre-commit run --all-files clean

(164 → 158 collected items is the six removed skip directives, which sybil
counted as items.)

AGENTS.md and the code-review skill documented the machinery this removes;
both now say to elide the varying tokens rather than guard the example.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (main@506de69). Learn more about missing BASE report.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #47   +/-   ##
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  Coverage        ?   98.30%           
=======================================
  Files           ?        5           
  Lines           ?      177           
  Branches        ?        0           
=======================================
  Hits            ?      174           
  Misses          ?        3           
  Partials        ?        0           

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