feat(callgrind-utils): add to_flamegraph SVG export

Add CallGraph::to_flamegraph / to_flamegraph_file mirroring the existing
to_json API, rendering a flamegraph SVG via the inferno crate.

To weight frames by cost, the parser now captures per-function self cost
and per-edge inclusive cost from the positions:/events: layout (first event
column, e.g. Ir); costs live outside Node so identity/dedup is unchanged.
redact() re-keys self costs onto redacted identities.

Folding walks roots top-down, distributing each function's aggregated self
cost across incoming paths in proportion to call inclusive cost; recursion
and cycles are terminated via an on-path guard and budget pruning.

Author: not-matthias

callgrind-utils/Cargo.lock

@@ -4,6 +4,7 @@ version = "0.1.0"
 edition = "2024"
 
 [dependencies]
+inferno = { version = "0.12.6", default-features = false }
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 thiserror = "2"

callgrind-utils/Cargo.toml

@@ -21,3 +21,14 @@ pub enum ToJsonError {
     #[error("I/O error: {0}")]
     Io(#[from] std::io::Error),
 }
+
+/// Errors raised while rendering a `CallGraph` to a flamegraph SVG.
+#[derive(Debug, Error)]
+pub enum FlamegraphError {
+    #[error("the graph carries no cost data (all self/inclusive costs are zero)")]
+    NoCost,
+    #[error("inferno flamegraph error: {0}")]
+    Inferno(String),
+    #[error("I/O error: {0}")]
+    Io(#[from] std::io::Error),
+}