fix: memory not released after indexing (20GB+ RSS for 5MB data)

[fxfxfx123]

Problem

After indexing a small project (65 files, 1.3MB, 2509 nodes), codebase-memory-mcp retains 20GB+ RSS on a 32GB Windows machine. Memory grows monotonically and is never released to the OS.

Root Cause (from source code)

1. mimalloc arenas from exited worker threads not purged - mem.c does not set mi_option_arena_purge_mult or mi_option_page_reclaim_on_free
2. Default worker count uses all 20 cores, each with its own mimalloc arena + 8MB stack
3. DEFAULT_RAM_FRACTION=0.5 means 16GB budget, no pressure to release

Fix

File	Change	Effect
src/foundation/mem.c	mi_option_arena_purge_mult=1	Purge arenas aggressively
src/foundation/mem.c	mi_option_page_reclaim_on_free=1	Reclaim pages from abandoned worker threads
src/foundation/mem.c	DEFAULT_RAM_FRACTION 0.5 to 0.25	Lower memory budget
src/foundation/system_info.c	Cap initial workers at 8	Reduce peak memory 60pct

Benchmark

Metric	Before	After
Threads	23	7
Peak RSS	20GB+	~2GB
Steady RSS	20GB+ growing	13MB stable
CPU	100pct	1pct

Tested on Windows 11, i7-12700, 32GB RAM, 65-file project.

[DeusData]

Thanks for the Windows memory-retention fix for #832. Triage: high-priority stability/performance PR.

Review will check the mimalloc options and worker-count change separately: we need post-index RSS to return to sane levels, but we also need to avoid an over-broad throughput regression or conflicting with the explicit memory-budget work in #752/#685. Please keep the before/after memory evidence current in the PR.

[fxfxfx123]

Yes, still active and waiting for review. All CI checks pass. Happy to adjust if there are concerns about the worker-count change — the mimalloc arena purge (mem.c) is the core fix, and the worker cap (system_info.c) can be dropped if it conflicts with #752/#685.