vm + cranelift: closure-bind HOF ctx forms via tree-bridge

[danieljohnmorris]

Summary

PR 3c in the HOF dispatch chain. The closure-bind variants of map, flt, fld, and srt (the 3-arg / 4-arg forms that thread an extra ctx value into every fn invocation) now run on VM and Cranelift, not just the tree interpreter.

Builds on #277 (native map), #278 (native flt / fld / flatmap), and #279 (tree-bridge for grp / uniqby / partition / 2-arg srt).

Repro

Before, this program errored on VM/Cranelift with UndefinedFunction because the dispatcher had no path for ctx-bearing HOFs:

look sym:t m:M t n>n;r=mget m sym;?r{n v:v;_:0}
prices pm:M t n syms:L t>L n;map look pm syms

main>L n;m=mset mmap "a" 10;m=mset m "b" 20;prices m ["a","b","a"]

After, all three engines produce [10, 20, 10].

What's in the diff

• vm: route closure-bind HOF ctx forms through tree-bridge adds (Map, 3), (Flt, 3), (Fld, 4), (Srt, 3) to is_tree_bridge_eligible. The existing 2-arg native arms short-circuit first, so the common shape still wins on the fast loop. Only the closure-bind forms pay the bridge round-trip, the same cost as 2-arg srt from PR 3b.
• test: cross-engine coverage for closure-bind HOF variants lifts the four happy-path tests in regression_closure_bind.rs (srt-by-lookup, map-with-lookup, flt-with-threshold, fld-with-external-accumulator) to run via run_all across tree, vm, and cranelift. Verifier-error tests stay tree-only because verify is engine-independent.
• example: lift vm/cranelift skip from closure-bind.ilo removes the two engine-skip directives. jit skip stays because jit lacks HOF dispatch entirely.

Test plan

• cargo test --release --features cranelift --test regression_closure_bind (8 passed)
• cargo test --release --features cranelift --test examples_engines (1 passed; closure-bind.ilo now exercised on vm + cranelift)
• cargo test --release --features cranelift full suite (2932 + every integration suite green)
• cargo fmt --check
• cargo clippy --release --features cranelift --all-targets -- -D warnings

Follow-ups

• PR 3d: direct FnRef-variable calls (f = dbl; f 10) lifted to OP_CALL_DYN from Expr::Call.
• PR 4: lift engine-skip: vm/cranelift from inline-lambda examples, now that FnRef + HOF dispatch work everywhere.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ All tests successful. No failed tests found.

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