fix: expand spread args to compiled Array.prototype.concat (#952)

[nickna]

Closes #952.

Problem

In compiled mode, a spread argument to Array.prototype.concat was not flattened:

console.log(JSON.stringify([0].concat(...[[1, 2]])));
// compiled: [0,[1,2]]   node: [0,1,2]

The array-method emitters built concat's argument array by emitting each AST arg via EmitExpression, but a spread ...arr is an Expr.Spread whose emit yields the inner array object — so the spread element reached the runtime ArrayConcat(list, object?[] items) as a single nested item, and concat's per-argument one-level flatten left it nested. [0].concat(b) (no spread) was always correct; it failed for boxed (non-numeric) arrays too.

The interpreter already expanded spreads correctly (EvaluateCall → GetIterableElements), so this was compiled-only, exactly as the issue stated.

Same family as the Math.max(...arr) → NaN gap (#951, PR #957), which introduced the helper this fix reuses.

Changes

Route concat's args through the existing EmitArgsArrayWithSpread helper (on IEmitterContext, added in #957) at every compiled dispatch path that emits Array.prototype.concat, so the spread is flattened before concat applies its per-argument flatten. The helper is identical to the old inline loop when no spread is present, so non-spread concat IL is unchanged → no regression.

• Compilation/Emitters/ArrayEmitter.cs — primary arr.concat(...) path (statically-typed array). Stash the receiver list, build the spread-expanded args array await-safely (the helper evaluates each arg into a temp with a clear stack, so the spread may contain await), then reload list under items. Excluded from the Compiled async-arrow write-capture emits invalid IL (PathStackDepth), even without shadowing #850 plain-arg pre-spill (!concatSpread) to avoid double-evaluating the args.
• Compilation/ILEmitter.Calls.AmbiguousDispatch.cs and Compilation/ExpressionEmitterBase.CallHelpers.cs — the two runtime-typed (string-or-list / any) dispatch paths for x.concat(...).
• Compilation/ILEmitter.Calls.cs — the shared argsArray case (Array.prototype.X.call), which also closes the same spread gap for reduce/slice/toSpliced/with via .call.

Tests

SharpTS.Tests/SharedTests/ArrayConcatSpreadTests.cs (new) — the issue repro plus flat-array / multi-array / mixed spread+plain / boxed-string / variable-source / empty / async spreads, run in both interpreter (the Node-matching oracle) and compiled modes.

Verification

• New tests: 16/16 green (8 cases × 2 modes); adjacent array/spread tests 156/156.
• Full suite: 14383 passed, 0 failed.
• The exact repro produces Node-identical output in both modes via the CLI.