engine: duplicate_unit diagnostics emitted once per stdlib module instance

[bpowers]

Description

Each custom unit (ghectare, hectare, pollution_unit, resource_unit) in World3 is reported as duplicate_unit ~20 times -- once per stdlib module instance that gets expanded during compilation.

Representative CLI output:

units error in model '' variable 'ghectare': duplicate_unit
units error in model '' variable 'hectare': duplicate_unit
units error in model '' variable 'pollution_unit': duplicate_unit
units error in model '' variable 'resource_unit': duplicate_unit

Note the empty model name -- the diagnostic is not attributed to any user model, which strongly suggests the duplicate-unit check is not model-scoped. The repetition count (20 instances for each of the 4 units) matches the number of stdlib module expansions in the model (smth1/smth3/delay1/delay3 sites).

Why it matters

• User-facing noise: ~80 duplicate_unit errors for a model whose units are actually declared exactly once. Obscures real diagnostics.
• Suggests an architectural layering bug in how stdlib modules share the parent project's unit list. If the parent's unit list is re-registered each time a stdlib model is built, the duplicate-detection check trivially fires -- which means the check is running at the wrong scope.
• Empty model '' attribution breaks error reporting downstream: a UI can't navigate to a variable in an unnamed model.

Components affected

• src/simlin-engine/src/db.rs (likely units_check tracked function and how it iterates across stdlib models)
• src/simlin-engine/src/units.rs (unit-context construction, duplicate detection)
• src/simlin-engine/src/stdlib.gen.rs (stdlib model expansion)

Possible approaches

1. Build the project-level unit context once and share it with stdlib module instances (via Arc, aligning with Make UnitMap cheaply cloneable via Arc to reduce per-context allocation overhead #318). Do not re-register user-declared units when compiling stdlib models.
2. Make duplicate detection idempotent: if a unit is re-declared with identical definition, accept silently; only flag conflicting redeclarations.
3. Scope duplicate_unit diagnostics to the model where the unit was declared; never emit them against stdlib model instances, which should inherit rather than declare.

Option 1 is cleanest and also reduces allocation overhead.

Context

Discovered while fixing the delay3 conflation bug in commit 6d48816. The real CLI surfaces 80 of these diagnostics on a model that declares each unit exactly once, making it a first-impression correctness problem on any model that (a) declares custom units and (b) uses stdlib smooth/delay modules.

Related: #35 (general unit checking), #318 (UnitMap Arc sharing).