feat(callgrind-utils): parse .out into a call graph with JSON and flamegraph output

New Rust crate (edition 2024) that reads a Callgrind .out profile and
extracts call-graph topology (costs/addresses ignored), serializing to
canonical index-ref JSON for stable cross-platform callgraph diffing and
to folded/flamegraph stacks.

Node identity is the {object,file,function} tuple so same-named statics
stay distinct. Edges are emitted only on calls= lines; name compression
across the three ID spaces, the cfl/cfi alias, inline fi/fe
callee-context inheritance, and multi-part merge are handled. A redaction
pass strips volatile addresses/paths so snapshots stay portable.

Fixtures are compiled and profiled by the in-repo Callgrind through an
rstest harness (vg-in-place, --instr-atstart=no plus client requests
keep loader/libc frames out): base graphs (recursion, chain, diamond,
mutual), arm64 unwind fixtures (tail calls, recursion, TLS access,
alloc/free cycles, longjmp, phantom recursion), the objskip seeding
underflow regression, and the arm64 TLS-descriptor regression.
Folded-stack snapshots and structural assertions; clippy and rustfmt
clean.

Author: not-matthias

callgrind-utils/.gitignore

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