Rework memory pressure system

[kwsantiago]

Closes #216

[Copilot]

Pull request overview

Reworks the node’s memory-pressure handling to trigger cache flushes more reliably (including during idle periods) and to reserve flush headroom for LevelDB allocations, addressing edge cases described in #216.

Changes:

• Reserve a fixed “flush headroom” in coins cache sizing to avoid running out of allocatable space during LevelDB batch writes.
• Add a cross-platform GetAvailableSystemMemory() helper and update SystemNeedsMemoryReleased() to use it.
• Change -lowmem defaults (200 MiB on Linux/Windows) and add a periodic idle scheduler task to flush caches when memory pressure is detected.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
src/validation.cpp	Reserves LevelDB flush headroom in cache sizing; integrates memory-pressure signal into flush decision and cache-emptying behavior.
src/util/mempressure.h	Exposes new available-memory query helper.
src/util/mempressure.cpp	Implements GetAvailableSystemMemory() and updates memory-pressure detection logic.
src/init.cpp	Updates -lowmem defaults/validation and adds periodic idle memory-pressure flushing via scheduler.

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