[starnix] Implement /proc/<pid>/smaps_rollup

Android tools (procrank, dumpsys meminfo, bugreport_procdump) inspect
/proc/<pid>/smaps_rollup for aggregate process memory.

Without it, tools fall back to parsing full /proc/<pid>/smaps. On
targets with many packages (e.g. Cuttlefish with full Android image,
350+ packages), traversing every VMA caused the DUMPSYS section in
dumpstate to exceed its 90s timeout (taking 96s+) and bloated
bugreport time to 8+ minutes in nested-VM CI.

This change:
1. Implements /proc/<pid>/smaps_rollup via ProcSmapsRollupFile,
   returning aggregate metrics in O(1) from cached MemoryStats.
2. Estimates Pss, Private_Dirty, and Shared_Clean using MemoryStats
   so diagnostics reflect realistic sharing instead of collapsing
   PSS to RSS.
3. Returns ESRCH on read() for tasks without an mm (zombies, kthreads),
   matching Linux 6.6 procfs behavior.

Benchmark on Cuttlefish with full Android userspace:
- DUMPSYS duration: 96.6s (timed out) -> 34.6s (0 timeouts)
- adb bugreport: 2m 53s -> 1m 32s

Fixed: 552681199
Test: fx test procfs_test zombie_test
Test: verified smaps_rollup and bugreport on Cuttlefish
TAG=agy
CONV=f5cfe3cb-60f9-48c9-8556-09a4e28af3f6

Change-Id: I3c1616b70482f42891359929d6fe4a18a41f75b7
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1781764
Reviewed-by: Robert Tarasov <tutankhamen@google.com>
Commit-Queue: Prashanth Swaminathan <prashanthsw@google.com>