fix: dereference typed pointers in VML models safely - #39

Fixes #25.

Changes

Automatically dereference typed pointers during VML model value and schema conversion, allowing expressions such as app.counter.count when counter is &Counter. Preserve the existing conversion behavior for scalar, struct, and array values.

Handle nil references without dereferencing them, and track ancestor addresses to avoid infinitely traversing cyclic object graphs while still permitting shared references. Schema conversion can inspect declared pointee types for nil references and empty pointer arrays. Include explicit handling for multiple pointer indirections supported by V 0.5.2.

Tests cover live values after actions, nested/shared/cyclic references, unused nil fields, nil-path diagnostics, empty and populated pointer arrays, invalid fields, scalar pointers, and multiple nil indirections.

Applications must still initialize references they intend to read (for example, counter: &Counter{}). This change does not allocate objects for nil pointers, make cyclic back-edges readable, or add pointer-target two-way writes.

Verification

Passed with V 0.5.2:

v -d ui2_headless test ui/vml_pointer_model_test.v
v -d ui2_headless test ui/vml_model_test.v

The regression fails against the original implementation and passes with this change. These commands also passed on Linux before branch publication: verification log. The shared run later stopped on an existing scrolling-test compilation failure for another issue.

Pointer regression tests also passed locally under AddressSanitizer (-cc gcc -gc none, leak detection disabled). No temporary verification infrastructure is included in this PR.