gc: per-frame memory-pressure detection + heap pinning + with_await fix

[borisbat]

Adds an opt-in tool to see per-frame GC pressure in imgui apps, pins the collectable heap, and fixes one per-command leak. Pairs with the daslang json PR (GaijinEntertainment/daScript#3034), which does the heavy lifting (frees dispatched snapshot trees, ?[] sentinel) — with_await here uses the new update() from that PR, so merge #3034 first.

Detection mode (imgui_boost_runtime)

Ported from strudel_player's heap-delta tracking:

• imgui_debug_memory(enabled) / imgui_reset_memory_baseline() / imgui_memory_report() — baseline + running max + a periodic per-frame line of heap / string-heap usage, gated behind a flag. Off by default — one bool test per frame when off.
• sample_memory() runs once per frame from begin_frame() (the universal per-frame reset); a teardown summary fires from harness_shutdown(), with a WARNING if heap or string-heap grew past 64 kb since baseline.
• imgui_memory_debug live-command (in imgui_visual_aids, the debug-toggle home) so the recorder / MCP can flip it remotely.

with_await leak fix

Rewritten to add the await modifiers onto the result object in place via the new update(), instead of copying every field into a fresh table and returning that — which orphaned the original JV(payload) wrapper on every await-wrapped command.

Heap pinning

options persistent_heap on imgui_boost_runtime + imgui_harness (collectable heap so per-frame JsonValue churn is reclaimed by delete + GC; runtime options unify program-wide).

Verification

• Live buttons.das run with imgui_memory_debug on: the steady heap held flat at ~0x21 kb across 5500+ frames, including the heavy imgui_snapshot dispatches at frames 2779/4856 — max briefly ticked to 0x26 kb on a snapshot then the steady heap returned, i.e. the ~40-widget tree is built and freed (the formerly-leaking path).
• Full integration suite: 171 passed headless; the 13 GLFW-backend synth tests (which need a real window) confirmed passing windowed — no regression.
• All changed files lint + format clean.

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