fix(expand): bound the total nodes produced by for-each

[LeadcodeDev]

Severity Low, category security. Location: crates/rustmotion-core/src/expand.rs:157

Impact

MAX_EXPANSION_DEPTH bounds how deeply directives may nest, not how many nodes they produce, and nesting one for-each inside another's template is explicitly supported (use_template_can_contain_a_nested_for_each, expand.rs:1005). Each nesting level costs exactly +1 depth (resolve_entry recurses with depth + 1 per produced node, while walk_children forwards depth unchanged), so up to 64 levels are legal and the output is the product of the array lengths. Four nested levels of 50 elements is 6.25M nodes from a file under 1 KB; eight levels of 10 is 10^8. Each iteration does a full Value deep-clone of the template plus a substitute walk, so the process hangs and OOMs during expand_directives — before validation, before any render, and with no diagnostic. An LLM-authored or third-party scenario reaches this through the documented public syntax; --no-validate is not needed.

Fix

Thread a mutable node/iteration budget through resolve_entry (e.g. a &mut u64 counter decremented per produced node, ceiling in the low millions) and fail with a named error when it is exhausted, exactly as ExpansionDepthExceeded does for depth. Cap items.len() per directive as a cheap first line of defence.

Evidence the audit read

/// Defense-in-depth ceiling on nested `use`/`for-each` expansion. ...
const MAX_EXPANSION_DEPTH: u32 = 64;
---
fn resolve_entry(... depth: u32) -> Result<Vec<Value>> {
    if depth > MAX_EXPANSION_DEPTH { return Err(...ExpansionDepthExceeded...); }
---
    let mut out = Vec::with_capacity(items.len());
    for (idx, element) in items.into_iter().enumerate() {
        ...
        let mut node = directive.template.clone();
        substitute(&mut node, &bindings, file_label)?;
        out.push(node);

Stacked on fix/media-protocol-allowlist, which carries the previous finding of this workstream. GitHub shows only this finding's diff; merge in order.

Part of the September 2026 audit remediation chantier. Refs #220 (RM-43).