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vm: fix array-compound regressions via OP_ARRAY_PRE_CHECK_LOCAL

Commit ab634827 introduced OP_ARRAY_COMPOUND_ASSIGN_LOCAL and
OP_ARRAY_INC_DEC_LOCAL but missed three AST corner cases:

 * assign_dim_op_undef — `$a[$b] += 1` on undefined $a should warn
   "Undefined variable $a" + "Deprecated null offset" + "Undefined
   array key", in that order. The native opcode evaluated the offset
   first and let arrayGet's "Trying to access array offset on null"
   replace the proper undefined-variable warning.

 * bug53432 — `$str[0] += 1` on a string should throw "Cannot use
   assign-op operators with string offsets" BEFORE the offset is
   evaluated. The native path was running arrayGet + arraySetLocal
   through the string-offset machinery instead.

 * bug70662 — when arrayGet's undefined-key warning triggers an error
   handler that creates the key, the compound write must be suppressed
   so the handler's value remains visible.

Adds OP_ARRAY_PRE_CHECK_LOCAL which runs BEFORE the offset is pushed.
It mirrors runArrayAccess.Run's ZtNull / ZtString compound-write-context
branches: warn + auto-vivify undefined containers, vivify explicit
nulls, and throw "Cannot use assign-op operators with string offsets"
for string containers. Warning order now matches the AST.

OP_ARRAY_COMPOUND_ASSIGN_LOCAL and OP_ARRAY_INC_DEC_LOCAL now also:
 * emit "Using null as an array offset is deprecated" in the read
   phase when the offset is null and the container is array/string/
   object (matches compile-array.go:712-717).
 * snapshot key existence pre-arrayGet and re-check the slot post-op.
   If the slot was replaced by a scalar (handler trashed it) or the
   key now exists when it didn't before (handler created it), the
   write is skipped (compile-array.go:826-862, bug70662 semantics).

Verified bit-identical to AST on assign_dim_op_undef, bug53432,
bug70662, plus the existing compound/incdec smoke tests.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

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