singleton: guard the state, not the program's name

The singleton lock protected a filename, not a store. It was keyed on
$EL_SINGLETON_DIR|$TMPDIR|/tmp + /el-singleton-<program>.lock — the
program's NAME and a temp directory — and never consulted the state it
claimed to protect, while its own refusal message read "Refusing to start
a second instance against the same state."

Measured, it failed in both directions. A second engram against a
DIFFERENT data dir was refused, naming the first's pid. And
TMPDIR=/tmp/other let a second engram start against the SAME data dir
with no complaint — the two-writer data-loss condition the guard exists
to prevent, defeated by one environment variable.

Both are one error: the identity of the resource had been replaced by a
label for it.

The lock now lives inside the state it guards —
<state>/.el-singleton-<id>.lock — and the program block says what that
state is. Same directory is the same file is the same inode, so it
contends and there is no TMPDIR left in the key to change. Different
directories are different files, so they don't. Different spellings of
one directory (trailing slash, x/../x, symlink) collapse in the kernel's
own path walk, so they contend without this code comparing strings;
canonicalisation is for the message, never the decision.

`guards:` is an expression so a program can point at the resolver that
already owns its path — guards: engram_resolve_data_dir() — instead of
restating that resolver's default, which is the two-owners defect spec
18.4 exists to prevent. A `singleton:` without `guards:` is now a compile
error; emitting a name-keyed lock instead would be emitting the defect.

Kept: the flock (the kernel drops it on crash and SIGKILL, so there is
still no "delete the lock file to get unstuck" ritual — a stale file
inside a copied data dir is inert), and the holder's pid in the message.
Changed: the message is true. It says "the same state" because the lock
it failed to take is in that state, and it names the state it checked.
An unguardable state (missing, read-only) now refuses rather than
starting unguarded.

Also corrects lang/AGENTS.md's compiler rebuild line, which had gone
stale: linking el_runtime.c alone no longer resolves.

lang/AGENTS.md

@@ -111,14 +111,18 @@ After changing any `.el` source in `el-compiler/src/` (run from the `lang/` dir)
 ```bash
 # 1. Stage2: current elc compiles the (modified) compiler to C
 ./dist/platform/elc elc-cli.el > elc-new.c
-# 2. Build the new compiler. The C link target is el_runtime.c — it holds the
-#    engram store + http/json/state impls the compiler output calls. el_runtime.c
-#    self-hosts elc on its own; el_seed.c is the (aspirational) seed layer and does
-#    NOT compile standalone under clang (missing prototypes for the el_runtime.c
-#    symbols it wraps — see caveat below), so link el_runtime.c here.
-cc -std=c11 -I runtime -lcurl -lpthread \
+# 2. Build the new compiler. Link the WHOLE runtime set, not el_runtime.c alone:
+#    el_runtime.c calls into engram_store / engram_vindex / eg_cosine_batch and
+#    wraps el_seed.c, so a one-file link fails at `ld` with undefined symbols
+#    (verified 2026-08-16 — the previous single-file line in this doc is stale).
+cc -std=c11 -O2 -I runtime -I$(brew --prefix openssl@3)/include \
+   -L$(brew --prefix openssl@3)/lib \
    -o dist/platform/elc-new \
-   elc-new.c runtime/el_runtime.c
+   elc-new.c runtime/el_runtime.c runtime/el_seed.c \
+   runtime/engram_cognition.c runtime/engram_geometry.c runtime/engram_reason.c \
+   runtime/engram_store.c runtime/engram_verify.c runtime/engram_vindex.c \
+   runtime/eg_cosine_batch.c runtime/eg_cosine_batch_strategy_cpu.c \
+   -lcurl -lssl -lcrypto -lpthread -lm
 # 3. Verify self-hosting FIXPOINT (stage3 == stage2 output, byte-identical):
 ./dist/platform/elc-new elc-cli.el > elc-verify.c
 diff elc-new.c elc-verify.c   # must be identical