Add OpenMP parallel declarations and iterable handling

[t-kalinowski]

Summary

• add OpenMP declare(parallel()/omp()) handling for for-loops and sapply assignments, plus toolchain flag detection
• extend sapply lowering to support vector iterables and inferred output shapes
• improve size-expression handling and iterable lowering; update docs and tests

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Fallback build drops requested OpenMP flags

When declare(parallel()) is used, compile() now sets use_openmp and feeds quickr_fcompiler_env() so that the first build uses OpenMP-specific Makevars (lines 219‑223). If that build fails because the toolchain lacks OpenMP support, the fallback system2() call immediately recompiles without env, so the shared object is produced with OpenMP directives treated as comments. This silently ignores the user’s request for parallelization and can make availability checks (e.g. openmp_supported_or_skip()) report success while loops run serially instead of erroring. The fallback should be disabled or the original failure propagated whenever OpenMP was requested.

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