fix accounting + report real macOS memory pressure (-P)

[aanogueira]

Summary

Fixes two memory-accounting bugs and adds an opt-in flag that surfaces the real macOS memory-pressure signal.

Bug fixes

• shared no longer misreports the compressor. It was printing compressor_page_count in the shared column — semantically wrong (Linux shared = Shmem, not compressed anonymous memory). macOS exposes no cheap system-wide shared-memory counter, so we now report 0 (honest placeholder) rather than a misleading value.
• speculative pages are now counted. speculative_count (read-ahead file cache) was never read, so it was missing from buff/cache and available. Including it tightens the total = used + free + buff/cache identity (was off ~0.5 GB, now ~0.4%).

New feature: -P / --pressure

macOS relieves memory pressure with an in-RAM compressor before it touches swap, so the Linux-style free/swap columns can look healthy while the machine is actually thrashing. -P prints the real kernel signal kern.memorystatus_vm_pressure_level (the same one Activity Monitor uses): normal / warning / critical, plus available %.

$ free -h -P
              total        used        free      shared  buff/cache   available
Mem:         32.0Gi      11.4Gi       7.4Gi          0B      13.4Gi      20.8Gi
Swap:            0B          0B          0B
Pressure: normal (64% available)

Default output stays Linux-parseable (flag is opt-in).

Validation

• make clean under -Wall -Wextra -Werror -pedantic
• clang-format --Werror clean
• make test smoke passes
• -P output cross-checked against sysctl kern.memorystatus_vm_pressure_level; column values cross-checked against vm_stat
• README updated

Follow-up (not in this PR)

A genuine shared value would require summing shared footprint across all tasks (as top/libtop do) — filed as a possible enhancement.

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