[C++] [bridge] `measure_handle`: definite-assignment analysis

Background

PR #4409 introduced a static isBoundHandle walk in lib/Frontend/nvqpp/ConvertExpr.cpp that diagnoses
bool b = h; when h was default-constructed. The check is intentionally narrow and has two known unsoundness gaps, documented inline as "defer to a follow-up.

Gap 1: function arguments

__qpu__ bool f(cudaq::measure_handle h) { return h; }

isBoundHandle short-circuits on BlockArgument SSA values (if (!load) return true;). The bridge silently accepts the discriminate even though the caller may pass a default-constructed handle. Solving requires either per-callee verification at every call site that the argument is bound, or a runtime sentinel check at every quake.discriminate.

Gap 2: aggregate / array per-element tracking

For measure_handle hs[N]; and struct Holder { measure_handle h; };, any store anywhere into the base alloca is treated as binding regardless of which element / member is loaded. Per-cc.compute_ptr-index reasoning needs SSA-value index resolution, deferred from PR #4409.

Start in lib/Frontend/nvqpp/ConvertExpr.cpp by reading the existing isBoundHandle walk and its inline notes about deferred gaps. Trace how BlockArgument values and aggregate or array allocas reach quake.discriminate, then define and validate sound handling for function arguments and per-element or per-member tracking.