Mark dead completion values at script/eval top level

[lahma]

Rebased onto latest main after #2605 merged.

What

A top-level statement's Normal completion value can never become the script/eval result when a
later sibling always produces one (an expression statement): nothing at top level can jump over
later siblings — break/continue cannot target past them, and throw abandons the value
entirely. Such statements now execute with completion-value elision on, which lets the for-statement
tight loop (#2605) engage at script/eval top level — the dromaeo-core-eval payload shape (a
string-append loop followed by ret = str;).

Soundness

The analysis applies only to Observable (top-level) lists, never to nested Inherit lists: a
labeled break out of a block skips the block's remaining statements while carrying an Empty value,
so an earlier statement's value can still surface through UpdateEmpty — a test pins that exact
shape. Only expression statements count as always-valued (untaken ifs complete with undefined
and would also qualify per UpdateEmpty(stmt, undefined), but the conservative set covers the
target shapes).

Results

EvalExecutionBenchmarks A/B (default job, adjacent runs):

row	before	after
EvalHotReusedEngine	732.7 µs	422.7 µs	−42.3%
EvalFreshEngine	718.0 µs	498.4 µs	−30.6%
NewFunctionHotReusedEngine (guard)	415.9 µs	416.9 µs	flat
PlainFunctionLoopReference (guard)	413.8 µs	420.6 µs	+1.6%
ParseOnlyTmp (guard)	38.7 µs	39.8 µs	flat

dromaeo-core-eval driver: 11.12 → 10.06 ms/iter (−9.5%). Allocations byte-identical on every row.

Eval bodies now hit the same per-iteration floor as function bodies, closing the historic ~13%
eval-vs-function execution gap. Composed onto the dromaeo-core-eval row (2 parses + eval-exec +
newFunction-exec): ≈ −20% on the row, to be confirmed at the next EngineComparison refresh.

Tests

Dead-marked top-level loop running tight with a trailing value read (script and eval), the
labeled-break-out-of-block shape that must NOT be dead-marked, and the spec undefined completion
of untaken trailing ifs.

Gating

All-TFM build ✅ · Jint.Tests ×2 ✅ · PublicInterface ×2 ✅ · Test262 99,260/0 ✅

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