fix: cap AttrValue nesting depth to prevent stack overflow on deeply nested config

[gominimal-aw-bot]

Routing-Key: inbox-route/I_kwDOSUhdos8AAAABMrkm-g

Evaluating a layer whose config nests lists, maps, or enum variants ~100+ levels deep aborted the entire process with an uncatchable fatal runtime error: stack overflow. AttrValue::from_term in the decode crate recursed through the List, Map, and EnumVariant cases with no depth limit, and Nickel forces one level of evaluation per recursive call, so deep nesting exhausted the stack before any error could be returned.

This caps attribute-value construction at 128 nesting levels and returns a new structured Error::AttrTooDeep once the cap is exceeded, turning hostile or accidental deep nesting into a recoverable error instead of a crash. A regression test drives evaluation past the cap on a large-stack worker thread and asserts the structured error rather than an abort. (The TOML argument path was also considered but is unaffected: the toml crate enforces its own nesting limit and returns a structured parse error.)

Verification

• cargo fmt --all --check — clean, no diff
• cargo clippy --workspace --locked -- -D warnings — exit 0, no warnings
• cargo build --workspace --locked — exit 0
• cargo test --workspace --locked — exit 0; decode: 57 passed, 0 failed (incl. nesting_past_cap_errors_instead_of_overflowing)

Note

Cap AttrValue nesting depth at 128 to prevent stack overflow on deeply nested config

Deeply nested Nickel attribute values could cause unbounded recursion in AttrValue::from_term, leading to a stack overflow. A MAX_ATTR_DEPTH constant (128) is introduced in attrs.rs, and recursion is tracked via a new from_term_at helper that returns Err(Error::AttrTooDeep { max_depth: 128 }) when the limit is exceeded. The new error variant is formatted and reported with a human-readable message in error.rs.

^{Macroscope summarized 97603c1.}

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