ci: disable incremental compilation, surface the nested build's error

Pipeline #8 failed and told us nothing. Both fixes here address that.

WHY IT FAILED: Woodpecker gives each workflow a fresh 10Gi workspace PVC. The
release gate alone fit (pipelines #5 and #6 passed), but the new `fast-suites`
step builds eight test binaries ahead of it, and the gate's nested
`cargo build --features fault-injection` then had no room. CARGO_INCREMENTAL=0
now applies to every Rust step: incremental artifacts are pure waste in CI since
nothing is ever reused across pipelines, and they measured 11G of a 27G target
tree locally — 41%.

WHY IT SAID NOTHING: tests/support/multiproc.rs:1686 built with
`stderr(Stdio::null())`, so the assert fired with "cargo build ... failed" and no
compiler error, no ENOSPC, no exit code. A build failure whose reason is discarded
costs more than the build. stderr is now captured and included in the panic
message; the output is only read on the failure path.

The env block is anchored on its first consuming step rather than a top-level
`variables:` key. I initially used the top-level form and reverted it: the file is
schema-validated and `when: branch: main` means only main triggers a pipeline, so
a rejected key could not be caught on a throwaway branch and would break every
push until reverted. Same rule the resource shapes already follow.

.woodpecker.yaml

@@ -33,6 +33,17 @@ when:
   branch: main
   event: [push, cron]
 
+# CARGO_INCREMENTAL=0 for every Rust step. Incremental compilation is pure waste
+# here: Woodpecker gives each workflow a FRESH 10Gi workspace PVC, so nothing is
+# ever reused across pipelines, and the artifacts are enormous. Measured locally,
+# `target/debug/incremental` was 11G of a 27G target tree — 41%.
+#
+# It is also what broke pipeline #8. The 10Gi workspace held the release gate
+# alone (pipelines #5 and #6 both passed), but once `fast-suites` built eight test
+# binaries ahead of it, the gate's nested `cargo build --features fault-injection`
+# ran out of room and failed with its stderr discarded, which is why the failure
+# said nothing (now fixed in tests/support/multiproc.rs).
+
 steps:
   # ── Fast in-process suites (push) ───────────────────────────────────────────
   # ADDED 2026-08-31 after a survey found that 14 of 23 integration suites were
@@ -57,6 +68,14 @@ steps:
         resources:
           requests: { cpu: "500m", memory: 1Gi }
           limits: { cpu: "2", memory: 4Gi }
+    # Declared ONCE here (first consumer) and aliased by every later Rust step —
+    # same technique as the resource shapes, and for the same reason: a custom
+    # top-level key would be a schema risk, and because `when: branch: main` means
+    # only main triggers a pipeline, a parse error could not be caught on a
+    # throwaway branch and would break every push until reverted.
+    environment: &cargo_env
+      CARGO_INCREMENTAL: "0"
+      CARGO_TERM_COLOR: never
     commands:
       - apt-get update && apt-get install -y --no-install-recommends protobuf-compiler cmake clang
       - cargo test -p tidaldb --lib
@@ -120,6 +139,8 @@ steps:
     # already sets it "for a shared CI runner" — and this is the step that BLOCKS
     # the image build, so its flake cost is the highest of any step in the file.
     environment:
+      CARGO_INCREMENTAL: "0"
+      CARGO_TERM_COLOR: never
       TIDAL_TEST_BOOT_BUDGET_SECS: "300"
       TIDAL_TEST_CONVERGENCE_BUDGET_SECS: "180"
     commands:
@@ -170,6 +191,8 @@ steps:
     # Same three-process shape as the release gate, so the same resource shape.
     backend_options: *resources-heavy
     environment:
+      CARGO_INCREMENTAL: "0"
+      CARGO_TERM_COLOR: never
       TIDAL_QUORUM_KILLPOINTS: "25"
       TIDAL_ELECTION_KILLPOINTS: "15"
       # Raised from 120/90 to match the release gate. These budgets were TIGHTER
@@ -221,6 +244,7 @@ steps:
       cron: nightly
     # In-process and fast; the light shape is sufficient.
     backend_options: *resources-light
+    environment: *cargo_env
     commands:
       - apt-get update && apt-get install -y --no-install-recommends protobuf-compiler cmake clang
       - cargo test -p tidal-net --test mtls -- --test-threads 1

tidal-server/tests/support/multiproc.rs

@@ -1675,7 +1675,12 @@ fn tidal_server_bin() -> PathBuf {
         // env var arms them — every non-fault suite spawns the same binary and is
         // unaffected). Production never passes this feature, so the shipped image
         // compiles the hooks out entirely.
-        let status = Command::new("cargo")
+        // stderr is CAPTURED, not discarded. It used to be `Stdio::null()`, and on
+        // 2026-08-31 this assert fired in CI with the message below and nothing else —
+        // no compiler error, no ENOSPC, no OOM, nothing to act on. A build failure
+        // whose reason is thrown away costs more than the build. The output is only
+        // read on the failure path, so a passing run is unchanged.
+        let out = Command::new("cargo")
             .arg("build")
             .arg("-p")
             .arg("tidal-server")
@@ -1683,12 +1688,16 @@ fn tidal_server_bin() -> PathBuf {
             .arg("fault-injection")
             .current_dir(workspace_root)
             .stdout(std::process::Stdio::null())
-            .stderr(std::process::Stdio::null())
-            .status()
+            .stderr(std::process::Stdio::piped())
+            .output()
             .expect("run cargo build");
         assert!(
-            status.success(),
-            "cargo build -p tidal-server --features fault-injection failed"
+            out.status.success(),
+            "cargo build -p tidal-server --features fault-injection failed \
+             (exit {:?}) in {}:\n{}",
+            out.status.code(),
+            workspace_root.display(),
+            String::from_utf8_lossy(&out.stderr)
         );
 
         let bin = workspace_root.join("target/debug/tidal-server");