kernel/vfs: raise the mount ceiling for N volumes; refuse (not drop) a full table (S3)

Multi-volume makes the mount table the bottleneck: each volume installs
one id-path mount and the system volume two FHS rewrites, so at the
volume manager's maximum_volumes (16) the old cap of 8 is far too low.
Raise maximum_mounts to 32 (headroom over the ~20-mount worst case) and
declare its bounds block; drop it from the bounds allowlist.

Fix a latent bug the higher pressure would expose: installMount silently
dropped a mount when the table was full, and mountBackend returned true
anyway — a full table was reported as a successful mount. installMount
now returns whether it placed the mount, and mountBackend propagates a
false so the mounting filesystem's harness logs "could not mount
<prefix>". At-limit is now a refusal that is observed, not a silent
success. (The full-table path has no host unit test: vfs.zig's tests are
not wired into the host aggregate — its import graph reaches the
freestanding kernel — so the correction rests on the propagated return
and the truthful bounds block.)

system/kernel/vfs.zig

@@ -55,7 +55,17 @@ fn tokenIndex(t: u64) u64 {
 
 // --- the mount table ---------------------------------------------------------
 
-pub const maximum_mounts = 8;
+/// bound: prefixes mounted in the kernel VFS table at once
+/// decided-by: ours
+/// protects: the `mounts` table below
+/// at-limit: refuse - installMount returns false and mountBackend propagates it;
+///   the mounting filesystem's harness logs "could not mount <prefix>" and the
+///   mount simply does not exist (no silent success). Budget: the initrd's
+///   top-level dirs (/system, /test) plus one id-path mount per volume and the
+///   system volume's two FHS rewrites — a few over the volume manager's
+///   maximum_volumes (16); 32 leaves headroom.
+/// observed-by: the harness "file-system: could not mount <prefix>" ring line
+pub const maximum_mounts = 32;
 const maximum_prefix = 64;
 const maximum_rewrite = 32;
 
@@ -156,7 +166,10 @@ pub fn setInitialRamdisk(image: []const u8) void {
     for (directories[0..directory_count], 0..) |*d, index| {
         const parent = parentOf(d.slice());
         d.parent = directoryIndex(parent) orelse index;
-        if (parent.len == 1) installMount(d.slice(), .kernel_initrd, null, "");
+        // Boot-time install of one mount per top-level initrd dir (/system, /test):
+        // provably few, far under maximum_mounts, so a full table here is
+        // impossible — but discard the result explicitly rather than assume it.
+        if (parent.len == 1) _ = installMount(d.slice(), .kernel_initrd, null, "");
     }
 }
 
@@ -167,7 +180,12 @@ fn directoryIndex(path: []const u8) ?usize {
     return null;
 }
 
-fn installMount(prefix: []const u8, kind: MountKind, backend: ?*ipc.Endpoint, rewrite: []const u8) void {
+/// Install (or remount-replace) a prefix. Returns false when the table is full
+/// and no slot could be claimed — the caller must surface that, never report a
+/// dropped mount as success. A remount of an already-mounted prefix reuses its
+/// slot and always succeeds; a /protocol remount is refused-as-noop (returns
+/// true: the first mount stands, nothing is dropped).
+fn installMount(prefix: []const u8, kind: MountKind, backend: ?*ipc.Endpoint, rewrite: []const u8) bool {
     // Remount replaces: a restarted backend re-mounts its prefix.
     var slot: ?*Mount = null;
     for (&mounts) |*m| {
@@ -176,19 +194,20 @@ fn installMount(prefix: []const u8, kind: MountKind, backend: ?*ipc.Endpoint, re
             // restarted FAT retakes /volumes/usb; letting it retake /protocol
             // would hand the whole naming layer to whoever asked second.
             // First mount wins, and init (PID 1) is always first.
-            if (std.mem.eql(u8, prefix, protocol_root)) return;
+            if (std.mem.eql(u8, prefix, protocol_root)) return true;
             if (m.backend) |old| ipc.dropRef(old);
             slot = m;
             break;
         }
         if (slot == null and !m.used) slot = m;
     }
-    const m = slot orelse return;
+    const m = slot orelse return false;
     m.* = .{ .used = true, .kind = kind, .backend = backend };
     @memcpy(m.prefix[0..prefix.len], prefix);
     m.prefix_len = prefix.len;
     @memcpy(m.rewrite[0..rewrite.len], rewrite);
     m.rewrite_len = rewrite.len;
+    return true;
 }
 
 // --- resolve -----------------------------------------------------------------
@@ -406,7 +425,7 @@ pub fn mountBackend(prefix: []const u8, backend: *ipc.Endpoint, rewrite: []const
             if (!isInitrdCarveOut(prefix)) return false;
         }
     }
-    installMount(prefix, .backend, backend, rewrite);
+    if (!installMount(prefix, .backend, backend, rewrite)) return false; // table full
     for (&mounts) |*m| {
         if (m.used and std.mem.eql(u8, m.prefixSlice(), prefix)) m.owner = owner;
     }