Phase 1: make every suite gate measurable #74

Why this phase is first

Three of the eight gate rows in docs/design/D-04-suite-roadmap.md cannot be improved blind, because nothing measures them:

Until these exist, every later phase can claim progress that no number supports. This phase buys the ability to measure; it changes no user-visible language behavior.

Scope

1. Official errorCode mapping for type and class diagnostics. psrs-typecheck's TypeCheckErrorKind already distinguishes TypeMismatch, OccursCheck, IntegerOutOfRange, NoInstance, OverlappingInstances, MissingInstanceMethod, AmbiguousConstraint, and FundepConflict, but error_code() returns None for all of them. Map the ones with an unambiguous purs counterpart. This alone makes 61 M4 cases and 92 M5 cases measurable.
2. A lenient type-check entry point. psrs_driver exposes check_program_lenient (resolution) and check_program_kinds_lenient; there is no type-check equivalent that tolerates missing support libraries, so L4/L5 cannot be scored against the corpus the way L2/L3 are.
3. L4 and L5 scoreboards, following the existing l2_resolution_scoreboard_with_annotations / l3_kind_scoreboard_with_annotations pattern: read @shouldFailWith, resolve leniently, report per-code agreement.
4. Harness module path. The scoreboard loads only a case's same-stem support directory and provides no library modules. Two consequences, both measured: 13 passing files fail because a case's other siblings in the same directory are not loaded (passing/Import/M2.purs, passing/Coercible/Lib.purs), and 230 fail purely on library modules. Putting stdlib/lib on the scoreboard's module path and loading a case's sibling modules from the containing directory is a harness-only change.
5. A runtime compile/run scoreboard for passing, and a warning-code scoreboard for warning.
6. A stated M8-O comparison. Decide what "optimize output agrees" means when the goldens are JavaScript, and record it.

Gate

Every row of the D-04 gate table carries a live measured number, and the L1–L3 numbers in that document are reproducible from the vendored corpus with no external dependency.

Sub-issues

Tracked as children of this epic.

References

• docs/design/D-04-suite-roadmap.md — Progress measurement, gate table
• crates/psrs-driver/tests/suite.rs — the L1/L2/L3 scoreboards to extend
• crates/psrs-typecheck/src/typecheck/error.rs — TypeCheckErrorKind and error_code
• crates/psrs-driver/src/program/lenient.rs — the lenient entry points

Validation

cargo fmt --all --check
cargo test --workspace
cargo clippy --workspace --all-targets -- -D warnings
PSRS_ORACLE=annotations cargo test -p psrs-driver --test suite -- --ignored --nocapture