Fixes memory leak on every node accessor

[redvers]

libxml2 has functions that return const xmlChar* / const char* /
const unsigned char* and must not be freed by the caller. Freeing
them either corrupts a libxml2-owned buffer (xmlBufferContent,
xmlOutputBufferGetContent) or attempts to free a pointer into the
caller's input string (xmlStrchr, xmlSplitQName3) or a parser-owned
name table (xmlParseName, xmlTextReaderConst*).

Authoritative source: meta/libxml2.xml (the castxml output) records
the C-level return type of every function. Pointer types _2682,
_2695, _2696 are const char*, const xmlChar*, const unsigned char* respectively. 53 libxml2 functions return one of
these; 42 of them are wrapped through this generator (the remaining
11 are ICU symbols ucnv_* / u_errorName_74 which the Pony binding
does not wrap).

Mechanism

Two typedefs in usetypes.xml: String emits an Xml2Free call after
the clone; String val does not. use.xml's per-function rv
attribute selects which typedef applies. castxml2pony emits
rv="String" for every xmlChar*-returning function regardless of
const-ness; the 42 const-returning entries are rewritten to
rv="String val".

Changes

meta/Makefile: castxml2pony_uses gains a sed loop over a
CONST_RETURN_FNS variable that marks each const-returning entry's
as rv="String val". The list lives in the Makefile so the
conversion replays on any future full regen from castxml output.

meta/uses/use.xml: 41 entries rewritten to rv="String val"
(xmlBufferContent was already done).

meta/functions.pony, meta/uses.pony, libxml2/raw/functions.pony,
libxml2/raw/uses.pony: regen output. The 40 newly exempted
wrappers now return String val and skip the free.

Tests

None of the newly exempted functions are called from the in-scope
Pony API. All 63 existing tests pass; stability confirmed across 4
consecutive runs. TestRepeatedCstringAccessors continues to
exercise xmlBufferContent through nodeDump.

If a future PR wraps any of the now-exempted functions, the Pony
API call site receives String val instead of String. That is
the correct shape for borrowed-from-libxml2 strings; it does not
break any existing caller.