Annotate unreachable and defensive code with LCOV exclusion markers

[mrabine]

Description

Several code paths are structurally unreachable or practically untestable, causing artificially low coverage metrics. Annotate these paths with LCOV_EXCL_LINE / LCOV_EXCL_START / LCOV_EXCL_STOP to reflect true coverage.

Acceptance criteria

• Constructor throw on allocation failure (malloc failures) annotated with LCOV_EXCL_LINE
• Defensive default: break and exhaustive switch branches annotated with LCOV_EXCL_LINE
• Defensive code unreachable annotated with LCOV_EXCL_START / LCOV_EXCL_STOP
• Improbable system error paths annotated with LCOV_EXCL_START / LCOV_EXCL_STOP
• Abstract class destructors and pure virtual stubs annotated