Fix environment iteration used by dmell export

[JaxCalder]

Problem

dmell saves variables through Dmod_SetEnv, which dmenv implements using its current context. Bare export lists names through Dmod_GetNextEnvName, but dmenv did not implement that API. The fallback iterator reads the host environment (or returns no names on embedded targets), so exported variables are missing.

Change

Implement Dmod_GetNextEnvName in dmenv using the same current context as Dmod_GetEnv and Dmod_SetEnv. Include inherited variables and omit names shadowed by a child context. Add regression tests for the export path and inheritance.

Verification

ctest --test-dir /tmp/dmenv-export-build -R '^test_dmenv_unit$' --output-on-failure — 27 tests passed.