fix(units): inherit units through alias chains, source-render if/match/struct diagnostics, drop stale report.json on type errors

[AkiraTamai]

Summary

Follow-up to #539/#541 for three issues found while testing the units checker via the CLI on develop.

1. Alias-of-alias lost its unit (bug). type Money = Usd; had unit: None, so m: Money + j: Jpy verified. New ModuleEnv::unit_of_type(name) walks _base_type until a unit tag is found (cycle-safe); units.rs and the proof cache both use it.

// module_env.rs
pub fn unit_of_type(&self, name: &str) -> Option<&String>  // Money -> Usd -> Some("USD")

The cache hash now includes name:base=unit for every alias that resolves to a unit, so re-pointing type B = A; to type B = J; invalidates cached results (unitless modules hash exactly as before).

2. AST Debug in unit diagnostics (cosmetic). expr_to_source_string fell back to {:?} for IfThenElse / Match / StructInit, producing IfThenElse { cond: Variable("c"), then_branch: Block([...]) ... }. Added source rendering: if c { A { amt: u } } else { B { amt: j } }, match x { Some(v) => { v }, _ => { 0 } }.

3. --json polluted by stale report.json (existing behavior, but unit errors make it easy to hit). Atoms rejected before Z3 (type/unit errors) never write report.json; the --json path then reads whatever file was left by a previous run and emits its top-level fields ("status":"success", "atom":..., "span":...) next to exit code 1. Fix: cmd_verify removes <report-dir>/report.json at start, so only this run's output can be read back. Also, type errors were tagged z3_sat (via z3_result_from_error_message(..).unwrap_or("sat")) although Z3 never ran; they are now tagged z3_skipped, same as unverifiable atoms. No Z3 encoding, MIR, or runtime changes.

Tests: tests/test_units.rs gains alias-chain positive/negative fixtures, an alias re-pointing cache-invalidation case, source-rendering assertions on the branch-mismatch fixtures, and a stale-report.json --json regression. docs/LANGUAGE.md notes alias inheritance.

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 4 potential issues.