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.)

tools/bounds-allowlist.txt

@@ -194,7 +194,6 @@ system/kernel/process.zig:write_buffer
 system/kernel/scheduler.zig:ipc_maximum_handles
 system/kernel/scheduler.zig:maximum_space_mappings
 system/kernel/vfs.zig:maximum_directories
-system/kernel/vfs.zig:maximum_mounts
 system/kernel/vfs.zig:maximum_prefix
 system/kernel/vfs.zig:maximum_rewrite
 system/services/acpi/acpi.zig:blocks