Function is_lisp_extension 

pub fn is_lisp_extension(path: &Path) -> bool

Predicate: assert that path terminates in the canonical LISP_SOURCE_EXTENSION (lowercase .lisp) — the file-type shape every M2 typed path-slot the wasm-engine instantiator reads as tatara-lisp source must take. The contract:

• the path has an extension component (no-extension paths like "lib/init" or "a" fail);
• the extension’s UTF-8 string form is exactly "lisp" — lowercase, no trailing residue, no double-extension shadow like ".lisp.bak".

Returns true on accept, false on reject. Each per-axis caller — crate::BehaviorSpec::validate on :behavior :on-* paths (c97815a), [crate::UpgradeInstruction::StateChange::validate] on :upgrade-from :state-change :script (this commit), every future axis admitting a tatara-lisp source path — wraps the boolean into its own typed *NonLispExtension { slot, path } / *NonLispExtensionScript { script } enum variant so the diagnostic still names which slot carried the non-.lisp value. The predicate is axis-agnostic; the wrapping per-axis variant carries the slot identity.

Lifted from caixa-core::behavior where the same single-line gate (path.extension().and_then(|ext| ext.to_str()) == Some("lisp")) was inlined verbatim across the first call site (BehaviorSpec::validate_callback_path) — the PRIME DIRECTIVE duplication-budget rule (THEORY.md §I.3.5: “every recurring shape becomes a generator before it becomes a pattern; every pattern becomes a library before it becomes duplicated code. The duplication budget is zero.”) promotes the gate to a typed substrate-side predicate on the same trajectory the path-shape gate is_sandboxed_relative_path already follows (lifted from the same two call sites once the second consumer appeared). The third caller — the future :bibliotecas per-entry tatara-lisp source-file axis (the feira build loop reads each through the same tatara_lisp::read reader at parse time), the future :exe :kind Binario entry-point axis (the nix-built binary’s entry point loads as Lisp source), the future M2.5 wasm-engine pre-warm hook axis — lands as a thin two-line wrapper rather than re-inlining the same extension check.

Pairs with the per-axis *NonLispExtension / *NonLispExtensionScript variants on crate::BehaviorError and crate::UpgradeError — those remain the typed surface authors see; this predicate is the single-source-of-truth gate the caixa-build pipeline consults to produce them.