Add mapr: short-circuit Result propagation across map

[danieljohnmorris]

Summary

Adds a new HOF mapr fn xs : R (L b) e that calls fn on each item, accumulates the inner Ok values on the all-Ok path, and short-circuits on the first Err. Mirrors Rust's collect::<Result<Vec<_>, _>>() pattern.

Originating friction: ilo_assessment_feedback.md line 2541 (html-scraper rerun3). The personas kept writing:

ton s:t>n;r=num s;?r{~v:v;^_:0}    -- swallow-err helper, ~30 tokens
ns=map ton xs                      -- and the error is lost

mapr collapses both lines to one and preserves the err shape:

ns=mapr num xs                     -- R (L n) t

Pair with ! to thread the err up into a Result-returning caller without per-item match boilerplate. The assessment doc says this "would remove the ton helper entirely" across every CSV/HTML-parsing program.

Repro before/after

Before:

$ ilo 'count xs:L t>R n t;ns=mapr! num xs;~len ns' --run-tree count '["1","2","3"]'
error: undefined function 'mapr'

After:

$ ilo 'count xs:L t>R n t;ns=mapr! num xs;~len ns' --run-tree count '["1","2","3"]'
3
$ ilo 'count xs:L t>R n t;ns=mapr! num xs;~len ns' --run-tree count '["1","bad","3"]'
^bad

Tree, VM, and Cranelift all agree byte-for-byte.

What's in the diff (per-commit)

1. builtins: register mapr discriminant, name, and tag (b1ec0fa) - enum variant, name table both directions, ALL list, round-trip test fixtures.
2. parser: mapr takes a fn-ref in slot 0 like other HOFs (5955b96) - parser knows to keep the first arg as a fn-ref (parallel to map, flt, flatmap).
3. verify: type mapr as R (L b) e from fn return R b e (2a86130) - derives the outer Result shape from the callback. Rejects non-fn first args and callbacks that don't return Result with ILO-T013 plus a hint to use map for non-fallible fns.
4. interpreter: implement mapr short-circuit Result propagation (6a41689) - tree-walker arm.
5. vm: route mapr through the tree bridge on VM and Cranelift (0c2be43) - (Builtin::Mapr, 2) joins grp/uniqby/partition/srt in is_tree_bridge_eligible, and tree_bridge_returns_result includes Mapr so ! propagation works.
6. test: cross-engine regression coverage for mapr (b645194) - 11 cross-engine cases covering happy path, empty, head/mid/tail Err, ! propagation, user-fn dispatch, and verifier rejections.
7. example: mapr happy/bail/bang/empty paths (546f081) - examples/mapr.ilo picked up by the examples_engines harness.
8. docs: document mapr in SPEC, ai.txt, and SKILL (95c59b5) - in-repo docs. Site reference is updated in ilo-lang/site#docs/mapr-builtin.

Test plan

• cargo test --release --features cranelift --test regression_mapr (11 tests, all engines)
• cargo test --release --features cranelift --test examples_engines (mapr.ilo runs across tree + vm with both out and err assertions)
• cargo test --release --features cranelift (full suite green; AOT tests need target/release/libilo.a, which is the standard CI layout)
• cargo clippy --release --features cranelift --all-targets -- -D warnings
• cargo fmt --all

Follow-ups

• Closure-bind ctx variant (mapr fn ctx xs) deliberately omitted; will add if a workload turns up that needs it.
• Site PR for the builtins reference page lives in ilo-lang/site on branch docs/mapr-builtin.

[codecov]

Codecov Report

❌ Patch coverage is 80.82192% with 14 lines in your changes missing coverage. Please review.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/interpreter/mod.rs	50.00%	14 Missing ⚠️

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