Proxy mesh variables inherit the swarm variable's units (#439)

[lmoresi]

Fixes #439.

The defect

SwarmVariable._create_proxy_variable did not forward units= to the proxy MeshVariable. Since var.sym resolves to the proxy, reading a proxied variable through the symbolic path returned the non-dimensional number with no units attached, presented as though it were the answer.

Measured, with density=3300 kg/m³ as the reference quantity and a stored non-dimensional value of 1.0:

	before	after
var.units	kg/m³	kg/m³
var._meshVar.units	None	kg/m³
var.array	3300.0	3300.0
var._meshVar.array	1.0	3300.0
evaluate(var.sym)	ndarray 1.0, no units	UnitAwareArray 3300.0 kg/m³

The two rows in bold are the bug: the swarm variable and its own proxy disagreed about the value, and evaluate handed back the raw non-dimensional number silently.

Stored data stays non-dimensional on both sides — only what the proxy advertises changes, so the transfer itself is untouched (asserted by test).

Context

This is the half of #426 that #434 deliberately left out. It was split off at the time because attaching units to the proxy makes var.sym unit-aware everywhere proxies are composed into expressions, which is a units-boundary decision rather than a coordinate fix. Maintainer decision was to forward the units and measure what moves.

Blast radius: none

• level_1 tier_a/tier_b: 571 passed (567 baseline + the 4 new tests), 0 failed
• level_2 tier_a/tier_b: 319 passed, 0 failed
• np=2 parallel swarm/proxy: 4 passed

No existing test both declares units on a swarm variable and reads it symbolically. That is why the change moves nothing — and also why the defect survived. Worth recording as a coverage gap in its own right, not just as a convenient result.

Tests added

To tests/test_0116_swarm_proxy_units.py (created by #434):

• proxy .units equals the variable's .units
• .array on both sides reaches the same dimensional number, while .data stays non-dimensional on both
• evaluate(var.sym) returns a UnitAwareArray with the right units and value — the silent-wrong-value case
• a variable declaring no units keeps a dimensionless proxy

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[lmoresi]

Adversarial review

"Blast radius: none" is a weaker result than it reads

571/319/4 passing means no existing test exercises the changed path at all, not that the change is safe. The condition to trip it is: a swarm variable that declares units=, with a proxy, read symbolically, under an active units model. Nothing in the suite does all four.

So the honest claim is "nothing regressed that is tested". Anything downstream that composed a proxied symbol and relied on it being dimensionless — arithmetic against a bare float, a constitutive term assembled without unit checking — now sees a unit-aware operand. The suite cannot tell us whether such code exists, and the docs/examples tree is not run by CI.

The place I would expect trouble is multi-material constitutive assembly, where a density proxy multiplies a level-set fraction. IndexSwarmVariable level sets carry no units, so that product is now unit-aware x dimensionless. That composes fine in principle; it is not exercised.

The type of _units being forwarded is not pinned

self._units is a pint.Unit by the time it reaches the constructor, not the string the user passed. MeshVariable accepts both — but the test asserts proxy.units == var.units, which would pass even if both were wrong in the same way. A test pinning the string ("kilogram / meter ** 3") exists only in the evaluate test, indirectly.

The vector/tensor case is untested

Every new test uses size=1. A vector swarm variable with units gets a proxy with self.shape components and one units= for all of them, which is presumably right, but "presumably" is doing work there. Same for dtype=int variables, where units are meaningless and now get forwarded anyway if declared.

What holds up

• The before/after is measured on both sides of the change rather than asserted, including the .array disagreement that is the concrete symptom.
• The dimensionless case is pinned, so the change cannot silently start attaching units to variables that never asked for them.
• The fix is one keyword at the single site that constructs the proxy; there is no second path to keep in step.