fix(semantic): surface DoubleSubject and MissingVerb errors (#603)

When encountering structural syntax violations such as `DoubleSubject`
(multiple nominatives fed as a subject) or `MissingVerb` (an assembled
sentence lacks an action but contains subjects or objects), the Semantic
Assembler silently allowed execution to proceed into the code generation
phase. This caused raw `rustc` compiler failures (like `E0425: cannot find value`)
to be surfaced to developers instead of the intended, idiomatic Greek
error messages (`Διπλοῦν ὑποκείμενον`, `Ῥῆμα οὐχ εὑρέθη`).

This commit adjusts `Assembler::handle_nominative` to immediately return
`AssemblyError::DoubleSubject` when a second standalone nominative is fed,
and `Assembler::finalize` to strictly assert `AssemblyError::MissingVerb`
for incomplete statements while preserving valid verbless fallback patterns
(like solitary queries, pure literal evaluations, or property access chains)
for tests. Tested via direct AST mocks and full integration runs.

Co-authored-by: madmax983 <1848860+madmax983@users.noreply.github.com>
Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>

Author: madmax983