Skip checkDebug()'s Python round-trip entirely in fast-continue mode

[revarbat]

Summary

• Found via a real BelfrySCAD bug report: debug + immediate Continue on a NURBS-surface script took ~2x longer than a plain render, even with per-function fast-continue already engaged.
• Measured root cause: 462,094 checkDebug() calls still crossed into Python even with fast-continue fully applied, because module/geometry evaluation never compiles (a separate, always-interpreted system) — every statement there still called into Python just to be told "continue". Disabling fast-continue entirely ballooned this to 5.99 million calls, confirming the existing feature was helping, just not eliminating the cost.
• Extends the same fast-continue mechanism to checkDebug() itself, not just VM eligibility: setFastContinueBreakpoints gains a second hookSkippable parameter — true only for a plain "Continue" with no step pending. step_over/step_out keep hookSkippable=false even though they also pass a real breakpoints set, since they still need every statement inspected for their own step_hit logic.
• Adds FastContinueSignal: a small, lock-free, GIL-free shared flag, since debug_evaluate() runs as one single blocking call with the GIL released for its duration — there's no live Python-callable Evaluator handle pause()/set_breakpoints() could otherwise interrupt directly. A caller creates one per debug session and calls .request() to guarantee the very next checkDebug() call isn't skipped.
• breakpoint() (forced=true) always bypasses the skip, unaffected.

Test plan

• New tests in test_debug_hooks.cpp: skip with no matching breakpoint, still-fires at a matching one, unaffected when not hook-skippable (step_over/out), interrupt flag forcing a call through, breakpoint()'s own bypass.
• Full suite: 1297/1297 passing.
• test_python_bindings.py: 13/13 passing, unmodified (new params have defaults).
• Verified end-to-end via the real Python binding: 20,007 → 1 hook call for a representative script with hook_skippable=True and no breakpoints; unaffected (20,007) with hook_skippable=False.
• Verified the interrupt flag: pre-arming it before a run starts forces exactly one extra hook call through, then correctly self-clears (test-and-clear).

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