test(e2e): assert local-index addressing on every host

Every test covering local-index addressing needed `gcc` or
`fresh-sandbox`, and Windows has neither capability. So the one platform
where path semantics actually differ was the one platform never
asserting them — which is why a fixture could write an MSYS path into
mcpp.toml and the failure surfaced four steps away, as "package not
found in any configured index", and was chased through the workspace
inheritance code that was not involved.

210 needs no compiler, no sandbox bootstrap and no network: it asserts
at the resolution layer, which is the layer that was broken. An absolute
[indices] path resolves; a workspace member inherits a root-relative one
and reads it; a miss in both reports the index as a READABLE root, since
"root absent" is the fingerprint of a path written in the wrong
spelling; and the route diagnostic that makes any of this findable stays
free of filesystem paths.

Author: speak-agent

tests/e2e/210_local_index_addressing_on_every_host.sh

@@ -0,0 +1,123 @@
+#!/usr/bin/env bash
+# requires:
+# 210_local_index_addressing_on_every_host.sh — a `[indices]` path must address
+# the same directory on every host, whether it is written absolute or inherited
+# from a workspace root.
+#
+# This exists because the tests that covered local-index addressing all needed
+# a compiler or a fresh sandbox, and Windows has neither capability — so the
+# platform where path SEMANTICS actually differ was the one platform never
+# asserting them. A fixture wrote an MSYS path into mcpp.toml, a native
+# mcpp.exe read the leading `/` as "root of the current drive", and the
+# resulting "package not found in any configured index" was chased for a day
+# through the workspace inheritance code, which was not involved at all.
+#
+# So: no compiler, no sandbox bootstrap, no network. Everything here is
+# assertable at the resolution layer, which is exactly the layer that was
+# broken. `mcpp add` is the probe because its existence gate reads through the
+# same routing `mcpp build` resolves dependencies with.
+set -euo pipefail
+source "$(dirname "$0")/_host_path.sh"
+
+TMP=$(mktemp -d)
+trap 'rm -rf "$TMP"' EXIT
+
+make_index() {  # make_index <dir>
+    mkdir -p "$1/pkgs/a"
+    cat > "$1/pkgs/a/acme.util.lua" <<'EOF'
+package = {
+    spec = "1",
+    namespace = "acme",
+    name = "util",
+    description = "local index fixture",
+    licenses = {"MIT"},
+    type = "package",
+    xpm = {
+        linux   = { ["2.0.0"] = { url = "https://example.invalid/u.tar.gz" } },
+        macosx  = { ["2.0.0"] = { url = "https://example.invalid/u.tar.gz" } },
+        windows = { ["2.0.0"] = { url = "https://example.invalid/u.zip" } },
+    },
+}
+EOF
+}
+
+# ── 1. an ABSOLUTE [indices] path ───────────────────────────────────
+make_index "$TMP/abs-index"
+INDEX_HOST="$(host_path "$TMP/abs-index")"
+mkdir -p "$TMP/abs-app"
+cat > "$TMP/abs-app/mcpp.toml" <<EOF
+[package]
+name = "absapp"
+version = "0.1.0"
+
+[indices]
+acme = { path = "$INDEX_HOST" }
+EOF
+cd "$TMP/abs-app"
+
+"$MCPP" add acme.util@2.0.0 > /dev/null || {
+    echo "FAIL: an absolute local index path did not resolve on this host"
+    "$MCPP" add acme.util@2.0.0 2>&1 | sed 's/^/  /'
+    exit 1
+}
+grep -qE '^util = "2\.0\.0"$' mcpp.toml || {
+    cat mcpp.toml; echo "FAIL: dependency was not written"; exit 1; }
+
+# A miss must report the index as READABLE. "root absent" here would mean the
+# path was addressed but does not exist — the exact fingerprint of a path
+# written in the wrong spelling.
+err=$("$MCPP" add acme.nope@1.0.0 2>&1) && {
+    echo "FAIL: expected a miss for acme.nope"; exit 1; }
+[[ "$err" == *"route: local index 'acme': root present, pkgs present"* ]] || {
+    echo "$err"
+    echo "FAIL: the absolute index is not being addressed as a readable root"
+    exit 1
+}
+
+# ── 2. a workspace member inheriting a ROOT-RELATIVE index ──────────
+# The member sees no [indices] of its own; the root's relative `path = "index"`
+# is anchored at the ROOT, not at the member's directory.
+mkdir -p "$TMP/ws"
+make_index "$TMP/ws/index"
+cat > "$TMP/ws/mcpp.toml" <<'EOF'
+[workspace]
+members = ["m1"]
+
+[indices]
+acme = { path = "index" }
+EOF
+mkdir -p "$TMP/ws/m1"
+cat > "$TMP/ws/m1/mcpp.toml" <<'EOF'
+[package]
+name = "m1"
+version = "0.1.0"
+EOF
+cd "$TMP/ws/m1"
+
+"$MCPP" add acme.util@2.0.0 > /dev/null || {
+    echo "FAIL: a workspace member could not read its root-owned local index"
+    "$MCPP" add acme.util@2.0.0 2>&1 | sed 's/^/  /'
+    exit 1
+}
+grep -qE '^util = "2\.0\.0"$' mcpp.toml || {
+    cat mcpp.toml; echo "FAIL: member did not inherit the workspace [indices]"; exit 1; }
+
+err=$("$MCPP" add acme.nope@1.0.0 2>&1) && {
+    echo "FAIL: expected a miss inside the workspace member"; exit 1; }
+[[ "$err" == *"route: local index 'acme': root present, pkgs present"* ]] || {
+    echo "$err"
+    echo "FAIL: the inherited index was anchored somewhere unreadable"
+    exit 1
+}
+
+# ── 3. the route diagnostic stays privacy-safe ──────────────────────
+# It is what makes a wrong path diagnosable at all, so it must be printed — and
+# it must not print the path, which is why it says present/absent instead.
+case "$err" in
+    *"$TMP"* | *"$(host_path "$TMP")"*)
+        echo "FAIL: the route diagnostic leaked a filesystem path"
+        exit 1
+        ;;
+esac
+
+echo "OK"