feat(units): track struct field units through let bindings, call results and nested access

[AkiraTamai]

Summary

Follow-up to #539 (review finding). The unit checker resolved x.field only when x was a parameter or result; let p = Price {..}; p.jpy + usd, mk_price(u, j).jpy + usd and q.price.jpy + usd silently passed as unitless.

support/units.rs now propagates a small type descriptor instead of a bare unit:

struct Ty { unit: Option<String>, struct_name: Option<String> }
vars: HashMap<String, Ty>          // was HashMap<String, String>
fn infer(&self, expr) -> MumeiResult<Ty>   // was -> MumeiResult<Unit>

struct_name is seeded from param/-> T types, Struct { .. } literals and callee return types, flows through let, if/match branches, and FieldAccess looks the field up on the inferred target type (so nesting works). Unit compatibility rules are unchanged; arrays remain unitless (documented in docs/LANGUAGE.md).

Fixtures: tests/test_units_struct_ok.mm (positive) and test_units_struct_mismatch_{let,call,nested}.mm (each rejected with Unit mismatch ... 'JPY' with 'USD'), wired in tests/test_units.rs.

Link to Devin session: https://app.devin.ai/sessions/2eef5afcd69d4e0ab19ada3b22d5cf7e
Open in Devin Desktop: https://app.devin.ai/desktop/session/2eef5afcd69d4e0ab19ada3b22d5cf7e?variant=devin
Requested by: @AkiraTamai

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Devin Review found 3 potential issues.