Postfix ++ and -- on an array element (4.2.24)

[ddehilster]

Bug. x[i]++ and x[i]-- ignored the index. They looked at the whole variable, saw several values, and failed with "Can't increment multi-valued variable", while the prefix form ++x[i] worked. The interpreter and compiled analyzers both had this gap. I found it while testing #778.

Fix. The indexed element is now changed, and its old value is returned, as postfix should. An index past the end pads with zeros, the same as assigning to x[i].

• Shared helpers: Arun::index_arg (find or pad the element) and Arun::bump_arg (±1 on a number or numeric string).
• Interpreted: the POSTUNARY case in Iexpr uses them.
• Compiled: Arun::postinc/postdec use them too. They also gain RFASem*-index overloads, so an index expression such as L("x")[L("i")]++ compiles. Prefix inc/dec already had these overloads.

Testing

• New fixture tests/postfix-index, with CI steps for Windows and Linux. It covers the returned value, the element changed, padding past the end, a numeric string, an expression index on a local array, a node variable's array, and plain scalars (unchanged). On 4.2.18 the same fixture produces no output and 5 errors.
• I also built the fixture as a compiled analyzer DLL; its output is identical to the interpreted run.
• All 40 Windows CI fixture steps pass locally, and the -DEV Full English Test output is unchanged.
• All 20 analyzers in analyzers/ (21 inputs) produce byte-identical output to 4.2.18.

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