Run the everyday make test on xdist workers

[sunt05]

What

make test now runs on pytest-xdist workers: -n auto --maxprocesses=$(TEST_JOBS) --dist worksteal, with TEST_JOBS defaulting to 4 and overridable (make test TEST_JOBS=8). The recipe's banner says so and points at pytest --lf for re-running failures. The test README gets the same note and stops linking a documentation page that does not exist.

Why

pytest-xdist has been a dev dependency since #1604 and the CI physics tier has used work stealing since then; only the local everyday run was still serial. On the same 1651-test selection, ten-core Apple silicon, CI wheel: 625 s serial against 124 s with workers, same results.

The cap defaults to 4 because each worker that draws one of the full-year DailyState tests materialises its own year of output (about 1 GB), so an unbounded worker count can exhaust a 16 GB laptop; a machine with memory to spare raises it per invocation.

Verification

• make -n test renders the intended pytest command.
• The 124 s figure is the measured make test selection with -n auto on this machine; the serial figure is the same selection without it.

No test content changes.

[github-actions]

CI Build Plan

Changed Files

CI/workflows (1 file)

• Makefile

Tests (1 file)

• test/README.md

Documentation (1 file)

• CHANGELOG.md

Build Configuration

	Configuration
Platforms	Linux x86_64
Python	3.12
Test tier	smoke (critical tests only)
PR status	Ready (standard matrix)

Rationale

• CI/workflow files changed -> validation build
• Test files changed -> validation build

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