Add systematic type-matrix and fuzz tests for expression functions

[davidkhk]

Summary

Systematic test-quality improvements for the Expression library: type-matrix tests across all five function categories, a property-based fuzz suite, and supporting infrastructure. No lib/ changes — test code, docs, and one CI workflow only.

Tests document current V1 engine behavior, they do not endorse it: where a function crashes on unexpected input, the test pins the exact exception (assert_raise + a # Known crash behavior, documented not endorsed: comment) so any future change is deliberate, not accidental.

What's included

• Infrastructure: stream_data dep; Expression.Test.TypeTestMatrix (canonical per-type sample values + evaluation helpers); Expression.Test.FuzzHelpers (StreamData generators + assert_no_crash); support modules compiled via elixirc_paths for :test.
• Type-matrix tests, one file per category (string, number, date, logical, enum) — each function probed against nil / numbers / strings / booleans / lists / maps / complex __value__ maps / unicode / edge cases. Pilot tests for upper, lower, abs, round, date.
• Crash-safe classification: the set of functions confirmed crash-safe lives once in Expression.Test.CrashSafe, consumed by both a deterministic classification test (runs in the normal suite, pins the set against the full type matrix on every CI build) and the fuzz suite (random exploration on top).
• Property-based fuzz tests (expression_fuzz_test.exs, tagged :fuzz, excluded from the default suite).
• TESTING.md documenting the approach, helpers, the context-coercion gotcha, and how to add tests for new functions.

CI: fuzz runs on a schedule, not as a merge gate

Property tests use a fresh random seed each run and exist to discover new crashing inputs, so they can legitimately go red — a poor fit for branch protection. .github/workflows/fuzz.yml runs them nightly (plus a manual "Run workflow" button) with FUZZ_MAX_RUNS=1000, so they never block PRs or develop merges. On failure, ExUnit prints the seed and StreamData prints the shrunk input; reproduce with mix test --only fuzz --seed <N>.

The schedule trigger only activates once this file is on the default branch. After merge it can be exercised immediately via workflow_dispatch.

Real defects surfaced (documentation-only here, pinned as current behavior)

• trim and first are not implemented — they return ERROR: "... is not implemented." strings despite being FLOIP functions.
• char/1 is byte-based, not codepoint-based — char(233) returns invalid UTF-8 <<233>> instead of "é", and disagrees with unichar.
• find / has_member crash on non-lists (Protocol.UndefinedError) while sibling enum functions degrade to error maps.
• to_string asymmetries between sibling functions (url_encode/url_decode, remove_first_word/remove_last_word).

Candidates for follow-up hardening; adding a function to Expression.Test.CrashSafe is the signal it's been fixed.

Verification

• Default suite: 711 tests + 560 doctests, 0 failures
• Fuzz suite: 20 properties, 0 failures — green at FUZZ_MAX_RUNS=1000 (20,000 evaluations)
• mix format --check-formatted clean · mix credo --strict exit 0 · MIX_ENV=test mix compile --warnings-as-errors clean