Add failing tests for env and free IMPORTANT audit findings

[kelp]

Summary

• Add 3 failing unit tests for env IMPORTANT audit findings
• Add 3 failing unit tests for free IMPORTANT audit findings
• Add integration tests for all 6 findings

env findings tested

1. -0 with utility not rejected: spec says both cannot be
   given together. Currently passes through to exec instead of
   erroring with exit 125.
2. Flags after NAME=VALUE still parsed: after a NAME=VALUE
   token, subsequent flag-like tokens should start the command.
   Currently -u after FOO=bar is still parsed as a flag.
3. -P sets PATH in child env: spec says -P restricts the
   search path for the utility, not the child's PATH variable.
   Currently env -i -P /path FOO=bar shows PATH in output.

free findings tested

1. -w wide mode buffers column always 0: MemInfo merges
   buffers+cached into buff_cache, losing the individual buffers
   value. Wide mode hardcodes 0 for the buffers column.
2. -c without -s silently succeeds: GNU free errors with
   "free: -c requires -s option". We silently display once.
3. -s short flag hijacked by si: argparse maps si field
   name[0]='s' as its short flag, so -s sets si=true instead
   of --seconds. Makes -s 1 -c 1 fail with "extra operand".

Test plan

• All 6 unit tests fail for the correct reasons
• No test hangs (verified with zig build test --summary all)
• No implementation code modified
• Phase 2: fix implementations to make tests pass