fix(tooling): cargo_binary rebuilds a stale binary, not only a missing one

core.process.cargo_binary built the Rust binary only when it was absent, a
behaviour inherited from the three bash wrappers it replaced (tooling/atlas,
generate-brands, generate-corporations). After an edit to the Rust source it
went on running the old binary.

For generate_brands this breaks the one promise the systems.db stamp makes.
`make regen-db` stamps the SHA of the NEW brand sources onto
generated_brands.toml produced by the OLD code, so the gate reports fresh and
the output is stale. It had already happened: the committed binary wrote
"Re-run: reach generate brands" while main.rs still said
tooling/generate-brands, so what was on disk had not been built from the
committed source.

It now always runs `cargo build --bin <name>` and lets cargo decide. An
up-to-date build is incremental and fast; a real rebuild is announced as an
event, since a silent ninety-second compile reads as a hang.

Proven on generate_brands (T-1253): with the old header restored, the rebuilt
binary reproduces generated_brands.toml byte-for-byte (sha256 e748531…). With
the new header it differs by exactly that one line.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tooling/core/process.py

@@ -147,34 +147,39 @@ def run(
 
 
 def cargo_binary(name: str, *args: str, capture: bool = True) -> str:
-    """Run a Rust binary from `server/`, building it first if it is absent.
+    """Run a Rust binary from `server/`, building it first if it is stale.
 
     Three scripts wrote this by hand — `tooling/atlas`, `generate-brands`,
     `generate-corporations` — each ~24 lines of identical bash: check for the
     debug binary, `cargo build --bin` it if missing, `exec` it with every
     argument. Three copies of one idea is a substrate, so it lives here.
 
-    Build-if-missing is kept because it is genuinely useful on a cold checkout,
-    but it is announced rather than silent: a first run that takes ninety
-    seconds with no explanation reads as a hang.
+    It ALWAYS asks cargo to build, and lets cargo decide whether anything is
+    stale. The copies all built only when the binary was MISSING, so an edit
+    to the Rust source ran the old binary — and for generate_brands that meant
+    `make regen-db` stamped the new source SHA onto output the old code
+    produced, the one thing the stamp exists to prevent (found T-1253: a header
+    change to main.rs did not reach generated_brands.toml). An up-to-date build
+    is incremental and fast; a real rebuild is announced, since ninety silent
+    seconds reads as a hang.
     """
-    binary = config.path("server", "target", "debug", name)
-    if not binary.is_file():
-        from tooling.core import console
+    from tooling.core import console
 
-        console.event(f"building the {name} binary (first run)", level="warn")
-        build = run(
-            ["cargo", "build", "--bin", name],
-            cwd=config.path("server"),
-            check=False,
-            missing_fix="install Rust — make setup-rust",
+    binary = config.path("server", "target", "debug", name)
+    build = run(
+        ["cargo", "build", "--bin", name],
+        cwd=config.path("server"),
+        check=False,
+        missing_fix="install Rust — make setup-rust",
+    )
+    if build.returncode != 0:
+        raise ReachError(
+            f"could not build {name}\n{(build.stderr or '').strip()}",
+            fix=f"cd server && cargo build --bin {name} — for the full error",
+            exit_code=build.returncode,
         )
-        if build.returncode != 0:
-            raise ReachError(
-                f"could not build {name}\n{(build.stderr or '').strip()}",
-                fix=f"cd server && cargo build --bin {name} — for the full error",
-                exit_code=build.returncode,
-            )
+    if "Compiling" in (build.stderr or ""):
+        console.event(f"rebuilt the {name} binary — its source changed since the last build", level="warn")
 
     result = run([str(binary), *args], check=False, capture=capture)
     if result.returncode != 0: