ILO-53: document and pin multi-statement function body chains

[danieljohnmorris]

Summary

• The parser already supported both inline (;-separated) and brace-block { stmt; ... } fn body forms; this PR makes that the official documented contract with a full regression gate
• Adds examples/fn-body-multistep.ilo exercising bind-chain, braceless-guard early return, ret-based early return, and Result unwrap mid-body (v=call!)
• Adds tests/regression_fn_body_multistep.rs — 18 cross-engine tests (VM + JIT) covering all three shapes the ticket requires
• Updates SPEC.md Functions section and skills/ilo/ilo-language.md with explicit body-form documentation

Closes ILO-53.

Test plan

• cargo test --test regression_fn_body_multistep — 18 tests pass (VM + JIT)
• cargo test --test examples — fn-body-multistep.ilo run/out annotations pass
• cargo test --lib — 3330 lib tests clean
• All existing single-expression body tests continue to pass

🤖 Generated with Claude Code

[codecov]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
688	1	687	0

View the full list of 1 ❄️ flaky test(s)

ilo::interpreter::tests::interpret_braced_guard_in_loop_no_early_return

Flake rate in main: 66.67% (Passed 1 times, Failed 2 times)

Stack Traces | 0.01s run time

thread 'interpreter::tests::interpret_braced_guard_in_loop_no_early_return' (32878) panicked at src/interpreter/mod.rs:10202:9:
parse errors: [ParseError { code: "ILO-P003", position: 27, span: Span { start: 35, end: 36 }, message: "expected `{`, got `;`", hint: Some("ilo bodies are single-line, `;`-separated — not python/swift-style indented. Use either the brace-block form `name p:t>r;{body1;body2}` or the single-line form `name p:t>r;body1;body2`. For statements that require a block (`@k xs{...}`, `wh cond{...}`, `?subj{...}`), the `{...}` must be on the same line as the head.") }]
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

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[danieljohnmorris]

needs manual rebase (conflicts in: tests/regression_fn_body_multistep.rs)

[danieljohnmorris]

needs manual — rebase conflict in non-doc file(s)