Add iterable for loops

[itsfuad]

Goal

Add direct iterable for loops after existing conditional and infinite-loop foundations.

Delivered

• Range iteration with exclusive bounds and deterministic common integral typing.
• Fixed-array, dynamic-array, and slice-view iteration.
• Value-only and index-plus-value binding forms.
• Target-usize sequence cursors and exposed indexes.
• Loop-lifetime shared sequence borrows with move, mutation, return, break, and continue safety.
• Canonical CFG init/header/body/latch/exit topology with definite-entry evidence for provably non-empty ranges.
• Structured HIR segments, MIR block-origin execution, exact cfg.SiteID cleanup, and LLVM lowering.
• Malformed-header recovery and valid-only semantic iteration evidence.
• Positive and negative source fixtures plus 386/amd64 backend validation.

Pull request

• Implement and harden for loops #124

Deferred

User-defined iterator protocol and optional-based next contracts remain separate work in #123. Direct arrays and slices stay statically lowered; no dynamic iterator dispatch was added here.

Validation

• Full uncached Go suite.
• Fresh bundled compiler.
• Full uncached x_test fixture suite.
• Linux 386 and amd64 LLVM validation through target clang.
• Diff and artifact checks.