Count the platform layer's traceability tags

`platform` was missing from the traceability scanner's source roots, so
every TRACES tag in `dr-plat` — the secret store, volume discovery, and
now display-profile acquisition — was invisible to the matrix. The
FR-PLAT-* family is exactly what that crate exists to satisfy, so the
omission understated coverage by the requirements it was meant to
count.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tools/traceability/src/main.rs

@@ -18,7 +18,12 @@ use anyhow::{bail, Context, Result};
 use traceability::*;
 
 /// Directories scanned for tags.
-const SOURCE_ROOTS: &[&str] = &["core", "ui", "apps", "tools"];
+///
+/// `platform` belongs here as much as the rest: it is where the display-server
+/// and secret-store integrations live, so leaving it out made every
+/// FR-PLAT-\* and NFR-PORT-\* tag in `dr-plat` invisible and understated the
+/// matrix by exactly the requirements the platform layer exists to satisfy.
+const SOURCE_ROOTS: &[&str] = &["core", "ui", "platform", "apps", "tools"];
 
 /// Minimum coverage the gate accepts.
 ///