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Walking skeleton: one let? shape end-to-end

Goal

End-to-end minimal slice: parser accepts let? (Some(x), rest) = t; as a block statement, typeck routes it through the general None-propagation rule, canon desugars it to a two-arm match, and one evaluator spec test runs green (debug). Proves the pipeline seam before any breadth work.

Implementation Sketch

Scope contract: the approved proposal (compiler_repo/docs/ori_lang/proposals/approved/fallible-let-binding-proposal.md) - Design + Semantics + Spec & Grammar Impact sections govern this section verbatim; no scope invention. TDD: the failing test matrix for this section’s shapes lands before implementation; debug AND release verified at close.

Spec References

  • compiler_repo/docs/ori_lang/proposals/approved/fallible-let-binding-proposal.md
  • compiler_repo/docs/ori_lang/v2026/spec/16-control-flow.md (16.0.3, 16.5, 16.6, 16.7)
  • compiler_repo/docs/ori_lang/v2026/spec/15-patterns.md; 13-variables.md; grammar.ebnf

Work Items

  • Land the minimal slice across parser -> typeck -> canon -> evaluator with one green spec test pinning let? (Some(x), rest) = pair; in an Option-returning function