test: Add 27 EXPR expression-language conformance fixtures

[leongdl]

Summary

27 new EXPR fixtures, 1 strengthened, 3 renamed, from a coverage audit of RFC 0005 / the published Expression Language spec. Suite: EXPR 350 → 379 passing, FEATURE_BUNDLE_1 55 → 57, 0 failing.

Areas covered

• Direct path property access on all four routes — direct {{Task.File.<n>.name}}, embedded with surrounding text, inside a function call, and Env.File direct. This is the exact shape of the validator bug fixed in openjd-rs PR #292; the only pre-existing fixture reached the property through a let binding, the one route that never failed. All four now green on both implementations.
• unresolved[T] progressive evaluation (zero prior fixtures mentioned it): valid op on an unresolved symbol passes check; type error on an unresolved value rejected at check; and the failing-arm suppression case — x if <unresolved bool> else <type-error arm> must validate, catching eager over-rejection.
• let scope must-rejects: step-template let referencing Session.WorkingDirectory / Task.Param.* / Task.File.*; environment-script let referencing Task.Param.* — every §3.6.2 must-reject cell was previously empty.
• Scope negatives inside expression trees (not just bare format strings): in-function-call and in-let variants.
• Whole-field null-drop: an EXPR-computed null drops an optional scalar field (FEATURE_BUNDLE_1 timeout — audited every @fmtstring annotation; it is the only optional scalar expression field), plus the non-null control.
• Operator precedence and associativity (zero prior fixtures): -2**2 == -4, 2**3**2 == 512 (right-assoc), a + b * c, not/and/or binding — 11 pins in one fixture.
• Literal-table rows: \xhh/\uhhhh/\Uhhhhhhhh/\N{name} escapes; uppercase 0X/0O/0B/R prefixes; 00 valid; 1e10 is float (discriminated via §1.3.4 pass-through); three misplaced-underscore rejects.
• Cross-type errors: 1 < "a", not 1, path(...)[0] rejected.
• IEEE floats: 0.1 + 0.2 == 0.30000000000000004 asserted (the spec's own worked example); 0.0/0.0 rejected; float("-inf") rejected.
• Session.HasPathMappingRules true branch via the runner's pathMapping: key — previously only ever asserted false.
• Boundaries: exactly-50 let bindings, 512-char identifier, leading underscore, forward-reference chain; int64-max accept twins for param default and supplied value; supplied 2^63 reject (verified green on BOTH implementations in a dual run).
• Strengthened: expr2.1.6--and-or-value-returning gained or-sentinel null discrimination and the false and x case.
• Renamed: three expr1.3.11--* fixtures → expr1.3.12--* (they test §1.3.12 range field extensions; §1.3.11 is Error Handling).

Companion PR

Five spec-correct fixtures that FAIL today (int64 param-default acceptance, the openjd-rs 2^62 false-reject, and others) are in the expected-failures PR (branch conformance-expr-lang-expected-failures).

[leongdl]

CI note: the two failing fixtures on the Rust leg (7.3--task-file-property-direct / -embedded) are expected against the RELEASED openjd-rs — they pin the Task.File property-access fix merged in openjd-rs PR #292, which has not shipped in a release yet. They pass against openjd-rs main. Separately, mainline CI is currently red for every PR due to two pre-existing broken fixtures (wrap-env-let-bindings-in-hooks, wrap-failed-enter-still-runs-wrap-exit) unrelated to this change.