agents: always write the memory cap, infinity for none

The cap file was removed for a cap of `infinity` or a RAM percentage,
so the boot check warned on every start of such an agent, for a
legitimate config. The file is now written on every start:

- a size: its byte count, as before;
- `infinity`: the literal `infinity`. The generators set no JSC heap
  ceiling and no subagent MemoryHigh=, and the boot check is quiet;
- a percentage: resolved on the host the way systemd resolves
  MemoryMax=N% for the container@ unit. That is permyriad (at most two
  decimals, at most 100%) of sysconf(_SC_PHYS_PAGES) *
  sysconf(_SC_PAGESIZE), rounded down to a page;
- a value systemd ignores (zero, over 100%, more than two decimals,
  unparseable) leaves the unit uncapped, so it is `infinity`.

The file is missing only when hive-c0re can't tell the cap: the
limits file and the kept drop-in are both unreadable, or a percentage
meets an unknown host memory size. The boot check now warns only for
a missing file or a first line that is neither a byte count nor
`infinity`.

Part of #5081 (slice S0-2).

nix/agent-modules/memory-cap.nix

@@ -3,10 +3,11 @@
 #
 # The cap is `MemoryMax=` on the host's `container@` unit, which nothing in
 # here can read: `/sys/fs/cgroup/memory.max` reads `max` at every level from
-# inside. hive-c0re writes its byte count to `/run/hive/memory-max-bytes`
-# before every start (`lifecycle::host_config`), and removes the file when the
-# cap is `infinity` or a RAM percentage. Two values are derived from it, and
-# neither is set without the file; `hyperhive-memory-cap-check` then logs a
+# inside. Before every start hive-c0re (`lifecycle::host_config`) writes it to
+# `/run/hive/memory-max-bytes` as a byte count, a percentage resolved against
+# the host's RAM as systemd resolves it, or `infinity` for no cap. Two values
+# are derived from a byte count, and neither is set for `infinity`. A missing
+# or malformed file sets neither too, and `hyperhive-memory-cap-check` logs a
 # warning at boot:
 #
 # - `BUN_JSC_forceRAMSize`, 75% of the cap. Bun/JavaScriptCore, the runtime
@@ -29,13 +30,15 @@
 # `checks.module-eval-agent-memory`.
 { pkgs, ... }:
 let
-  # Sets `bytes`, or runs `noCap` when there is no file or its first line is
-  # not a plain byte count. A leading zero would read as octal, and 16 digits
-  # or more could overflow `bytes * 75`. Shell builtins only.
+  # Sets `bytes`. Exits for `infinity`, and runs `noCap` when there is no file
+  # or its first line is neither `infinity` nor a plain byte count. A leading
+  # zero would read as octal, and 16 digits or more could overflow
+  # `bytes * 75`. Shell builtins only.
   readCap = noCap: ''
     bytes=
     [ -r /run/hive/memory-max-bytes ] && read -r bytes < /run/hive/memory-max-bytes
     case "$bytes" in
+      infinity) exit 0 ;;
       "" | 0* | *[!0-9]* | ????????????????*) ${noCap} ;;
     esac
   '';
@@ -55,16 +58,16 @@ let
   '';
 
   # The generators run before the container's journal starts and an nspawn
-  # container has no /dev/kmsg, so the boot warning for an unusable cap comes
-  # from a unit. `<4>` is the journal's warning priority.
+  # container has no /dev/kmsg, so the boot warning for a missing or malformed
+  # cap file comes from a unit. `<4>` is the journal's warning priority.
   check = pkgs.writeShellScript "hyperhive-memory-cap-check" ''
     ${readCap ''
       if [ -e /run/hive/memory-max-bytes ]; then
-        why="does not start with a plain byte count"
+        why="holds neither a byte count nor infinity"
       else
         why="is missing"
       fi
-      echo "<4>/run/hive/memory-max-bytes $why: no BUN_JSC_forceRAMSize heap ceiling and no MemoryHigh= on hive-subagent-daemon; the host's MemoryMax= on the container still applies. hive-c0re writes no file for a cap of infinity or a RAM percentage." >&2
+      echo "<4>/run/hive/memory-max-bytes $why: no BUN_JSC_forceRAMSize heap ceiling and no MemoryHigh= on hive-subagent-daemon; the host's MemoryMax= on the container still applies." >&2
       exit 0
     ''}
   '';