feat(runtime): configurable garbage collector, memory leak fix, regression test

[zaccharyfavere]

Adds a configurable GC sweep mechanism for orphan reactive resources, exposes Skip persistent memory (from palloc.c) usage as a metric, fixes a separate memory leak in the reactive read tracking, and ships a regression test that detects future memory leaks.

GC configuration:

• New GCConfig type and SkipService.gcConfig?: () => GCConfig in the public API
• The config lives in user TypeScript as a lambda; the runtime invokes it via FFI each time it is needed, with no caching
• New SkipRuntime_callGCConfigProvider FFI bridge (Skiplang -> JS via C++) following the same pattern as Mapper__mapEntry
• checkGarbage() sweeps expired resources from the garbage queue and evicts the oldest entries when the queue exceeds maxGarbageSize
• getGarbageQueueSize() returns the current queue depth

Memory metric:

• New SKIP_get_persistent_size() in palloc.c (64-bit) aligned with the existing 32-bit version in runtime32_specific.c
• Exposed through the full FFI chain (Skiplang prelude binding, Runtime.sk wrapper, C++/WASM bindings, public TS method getSkipPersistentSize())

Reads leak fix:

• Context.reads (SortedSet) accumulated read paths from root-level operations (createReactiveResource, closeReactiveResource) that were never released, unlike the per-mapper save/restore mechanism
• The set is now cleared at the end of each successful sweep in checkGarbage()

Regression test (skipruntime-ts/examples/stress-tests/gc-stress.ts):

• Default mode (~30s) runs two tests on the two leak scenarios
• Full mode (-f) runs longer, exhaustive versions
• It can display its trace and what it tries to do with the --trace option
• Computes a linear regression slope on memory measurements taken during a stable phase (after skipping the warmup window)
• Fails if the slope exceeds 100 bytes/cycle threshold
• Uses getSkipPersistentSize() for byte-precise measurement
• Wrapped by a Makefile target: make test-mem-stress, with args pass-through via ARGS="--trace --full"

Notes:

• The DeletedDir leak fix is in a separate PR
• The "instantiate-close-with-map" sub-test currently FAILs on this branch alone; it passes once the DeletedDir PR is also merged