refactor: real_fn! macro + fix PTS fd leak (#35)

[Bahtya]

Summary

• Replace 78 manual std::mem::transmute(real("sym\0")) call sites with a real_fn! macro that centralizes dlsym resolution and adds debug_assert! null checks
• Fix Vfd::Real fd leak in remove_table_and_get_fds() — PTS slave fd was never closed, causing stress: 20 rapid bash sessions to fail at iteration 4 (PTY exhaustion)
• Net reduction: 121 additions, 252 deletions (−131 lines)

Changes

src/preload.rs — real_fn! macro

• New real_fn!("sym\0", Type) macro: calls real(), debug_assert! null check, transmute to target type
• All 78 transmute+real call sites replaced (27 unique symbols, 16+ type signatures)
• Multi-call sites (poll, read, write, chdir, etc.) resolve once and reuse the local variable
• Existing wrappers like real_close(fd) kept with updated internals
• 2 fdsan transmute calls intentionally untouched (different pattern)

src/vfd.rs — PTS fd leak fix

• remove_table_and_get_fds() now also collects Vfd::Real fds (e.g. PTY slave on fd 0/1/2)
• Deduplicates by real fd value (since 0/1/2 share the same pts fd)
• Previously only collected Vfd::File fds, causing 1 fd leak per hermux session

Test results

• 31/31 APK tests pass (was 30/31 — stress: 20 rapid bash sessions now passes)
• 43 unit tests pass, 0 clippy warnings
• No performance regression (all timing tests within ±5ms noise)

Closes #35

Bahtya