fix(installer): macOS memory floor is warn-only and VMs are never sized (colima hard-coded 6 GB)

[LukasWodka]

Why

On macOS the memory picture is the Windows WSL2 story with weaker guardrails:

• The hard floor is effectively unenforced: on Mac the floor check only warns (Linux hard-fails), and the post-Docker runtime recheck is explicitly WARN-only — a Docker VM below the 5 GB floor proceeds and OOM-crashloops the client.
• Recommendations aren't clamped to physical RAM (PF_REC_MEM_GB=16 → "raise to 16 GB" on a 16 GB or smaller Mac is impossible or nonsensical).
• Nothing ever sizes a VM: Docker Desktop's VM is only hinted at; colima is hard-coded --memory 6 — too big for a ≤8 GB Mac to spare, never derived from hw.memsize.

Where (v1.9.6)

• Warn-only floor + recheck: scripts/lib/preflight.sh L234–238, L264–276; floor/rec constants L26–28.
• colima hard-coded: scripts/lib/setup-macos.sh L74.
• Reporting flip-flop (host vs Docker-VM label): scripts/lib/preflight.sh L147, L227, L235 — reporting side tracked in fix(installer): memory check flip-flops between WSL VM budget and physical RAM; impossible recommendations #417.

Scope

• Enforce the runtime-budget floor on macOS (fail with the exact fix, or auto-size with consent).
• Size colima from hw.memsize (e.g. min(half of physical, recommended), floor-checked); clamp every recommendation at physical − 2 GB.
• With consent, set Docker Desktop's VM memory (settings.json memoryMiB) the way the Windows .wslconfig ticket does — one shared sizing helper.

Acceptance criteria

• A sub-floor VM can no longer sail through preflight; no recommendation exceeds physical RAM; fresh colima on an 8 GB Mac starts with a workable, derived budget.