fix(memory): add MADV_WIPEONFORK to os_lock on Linux

[jgowdy-godaddy]

Summary

• Adds MADV_WIPEONFORK hint to locked pages on Linux so forked child processes have the pages automatically zeroed rather than inheriting key material
• Already had MADV_DONTDUMP; this is the matching fork-safety primitive
• Best-effort (like DONTDUMP) — kernel silently ignores it if unsupported

Test plan

• cargo test --workspace --all-targets passes (all existing memory tests pass)
• No consumer breakage: zeroing on fork is always correct for key material; no legitimate use case needs forks to inherit locked secrets