[P2] Valid conditional blocks can underflow the operand stack #15

Reproduced during the 2026-09-04 language review at a20753c6cff96ee86758744e227a04460234a832. Priority: P2.

let x = if true {
    1
} else {
    2
}
x

This passes canonical checking but fails at execution with pop_stack_value on empty stack and missing-x diagnostics. The compact legal version using 1; and 2; behaves the same. let x = if true 1 else 2; x succeeds and returns 1.

Canonical newline lowering inserts VM semicolons. The inherited conditional-block code preserves its tail only when last_was_sep is false; see IfTrueBlock. Canonical try/catch received special tail-preservation handling, but this conditional path did not. The authoring guide explicitly recommends conditional values, making this more than an undocumented style preference; see authoring-for-llms.md.

Specify whether each block context produces a value, then preserve it during lowering. Every valid expression branch should leave exactly one operand. Empty or statement-only branches must either yield a specified nil or be rejected in value position. Test if, elif, and else inside assignments, arrays, calls, and try; test LF, CRLF, and explicit semicolons. Function blocks also need an explicit return rule: current canonical tails are discarded unless return is used.

Acceptance: implement the described correction, publish the resulting language contract, and add focused regression coverage including applicable capability, streaming-parser, and resource-limit cases.

Fix and executable regressions: PR #19. Workspace tests, explicit vendor suites, formatting, strict Clippy, and the Linux ARM64/RISC-V and Windows GNU compile checks pass locally. All GitHub Actions checks pass on f1e775f, including Linux tests, fuzzing, AddressSanitizer, and Miri. This issue remains open until the fix is merged.