Fix every memory defect the checker found, and bump to 0.4.0

Six findings, all of them libakgl's, all closed. The memcheck run is clean.

Not one of the four json_load_file calls in src/ was ever matched by a
json_decref, so every asset load abandoned its parsed document: 1.5 KB per
sprite, 2.1 KB per character, 9 KB per load of the 2x2 fixture map, and on
the order of a megabyte for a real one. Each loader now releases it in the
CLEANUP block it already had, on the success path as well as the failure
one. akgl_registry_load_properties needed its loop moved inside the ATTEMPT
block first: props is a borrowed reference into the document and is read
after the block ends, so every exit from that loop leaked the whole tree.

akgl_get_property copied a fixed AKGL_MAX_STRING_LENGTH bytes out of what
SDL handed back, which is SDL's strdup of the value -- four bytes for
"0.0". That read up to 4 KiB past the end of somebody else's allocation on
every property read, returned whatever was there past the terminator, and
would have faulted on a value that landed at the end of a page. It copies
the value and its terminator now, and refuses a value too long for an
akgl_String rather than truncating it into an unterminated buffer. The
header note that described the overread as a quirk describes correct
behaviour instead.

The four savegame name tables wrote a fixed-width field starting at the
registry key, and SDL sizes that allocation to the name. They read past it
on every entry and put what they found into the save file: up to half a
kilobyte of this process's heap per registered object, in a file a player
might send to somebody. They stage through a zeroed buffer now, and a
negative-array-size typedef fails the build if a table's width ever outgrows
it.

akgl_controller_list_keyboards never freed the array SDL_GetKeyboards
allocated for it.

A font could be opened and published and never handed back -- there was no
way to close one, so a game that changed fonts between scenes leaked ten
kilobytes each time, and loading over a live name leaked the font it
displaced. akgl_text_unloadfont is that missing half, and akgl_text_loadfont
calls it when it replaces a name, after the new font has opened so a failed
reload leaves the caller with the font they had. A new public symbol takes
the version to 0.4.0 and the soname with it: an 0.3 consumer cannot be
handed this library and told it is the same ABI.

tests/registry.c fills a destination with a sentinel and asserts the bytes
past the terminator survive a read, which fails against the old copy.
tests/text.c covers unload, double unload, unloading a name that was never
registered, and replacement closing the displaced font. The JSON releases
have no test of their own and cannot sensibly have one -- nothing in the
public API can observe a jansson refcount -- so the memcheck run is their
test, which is an argument for gating it rather than against.

The two remaining findings are in deps/semver's own unit test, which is
vendored. They are suppressed by function name, so a rewrite of those cases
comes back as a finding.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

scripts/valgrind.supp

@@ -54,3 +54,32 @@
    ...
    fun:FT_Init_FreeType
 }
+
+# deps/semver's own unit test leaks the buffers it cuts strings into --
+# semver_unit.c:8 and :21, one calloc each per case. Vendored code, and its test
+# program at that: not ours to edit, and not worth carrying a local patch for.
+# Named by function rather than by file so that a rewrite of those cases stops
+# being suppressed and comes back as a finding.
+#
+# This is the only entry here that hides a real leak in a program this build
+# runs. It is bounded, it is in a test, and the alternative is either a fork of
+# a vendored dependency or a memcheck run that is always red -- see TODO.md,
+# "Memory checking" item 39.
+{
+   vendored-semver-unit-test-strcut-first
+   Memcheck:Leak
+   match-leak-kinds: definite
+   fun:calloc
+   fun:test_strcut_first
+   fun:test_strcut
+   fun:main
+}
+{
+   vendored-semver-unit-test-strcut-second
+   Memcheck:Leak
+   match-leak-kinds: definite
+   fun:calloc
+   fun:test_strcut_second
+   fun:test_strcut
+   fun:main
+}