lang: give cross-cutting concerns an owner instead of a convention

El's units of encapsulation are the function and the module. Neither can hold
a concern that belongs to the process, so each one had been expressed the only
way it could be -- as a convention: call this at every site. Conventions of
that shape do not hold. Measured here: zero process-identity guards at any
layer, 20 environment variables each with its default written inline at the
read site, 62 persist call sites, 10 per-route auth checks. One absence, four
times.

Step 0 first, because the premise was wrong. El was believed to have no
middleware or effect mechanism. It has one, and it is already load-bearing:
codegen injects engram_boundary_beat at the entry of every @manager/@accessor
fn, decorators take arguments and stack, dharma_emit from a non-@manager fn is
a #error, and the cgi block injects el_cgi_init at the head of main(). So the
correct move was not to invent a mechanism but to generalize the seam that
already existed. The real gap is narrower and is now recorded: the seam is
prologue-only and its callee is a fixed builtin.

Adds a `program` block -- the third program-level declarative block. cgi and
service declare what a program may do; program declares what it is.

  program "engram" {
      singleton: "engram"
      env ENGRAM_BIND: String = ":8742"
      env GUIDE_PORT:  Int    = "8771"
  }

singleton takes an exclusive flock before any user statement runs and refuses a
second start, reporting the holder's pid. It is a lock rather than a pidfile so
the kernel releases it on death including SIGKILL -- no stale state, and so no
"delete the lock file to get unstuck" ritual, which would itself be a
convention. It reports the pid because "already running" is not actionable; a
pid is. That is the direct answer to a stale process surviving a pkill and
going on answering probes.

env entries resolve once at startup -- environment wins, declaration supplies
the fallback -- and validate as a whole, reporting every problem at once rather
than costing one restart per variable. config("X") for an undeclared X is
fatal, because an advisory schema is just another convention. Programs without
a program block are unaffected, so migration is per-program.

Only one keyword is added. `config` and `env` could not become keywords -- both
are real identifiers in the tree -- so the block's fields are read as
identifier token values by its own parse loop and stay usable everywhere else.

The init function is emitted at the block site and called from main() rather
than inlined into main(). The live backend is codegen_streaming, which emits in
source order and cannot hold the entry list alive until main(); this way only a
single bool has to survive.

Also fixes: config() was defined in el_runtime.c but never prototyped in
el_runtime.h, so any el program calling it failed to compile under C99.

Spec: section 18 documents what shipped. Section 9 is corrected -- it claimed
decorators had no structural meaning, which has not been true for some time.
Section 19 designs durability-as-an-epilogue-effect and route authorization
and states plainly why neither is implemented here: both land in files under
concurrent modification, and the prerequisite for both is lifting the seam
from prologue-only to prologue/epilogue.

Self-hosting fixpoint verified byte-identical.

lang/runtime/el_runtime.h

@@ -957,6 +957,22 @@ el_val_t  __url_decode(el_val_t s);
 /* Environment */
 el_val_t  __env_get(el_val_t key);
 
+/* Cross-cutting concerns declared by a `program` block (spec §18).
+ * All three are COMPILER-INJECTED at the head of main() — they are not meant to
+ * be written by hand, which is the point: the guarantee cannot be forgotten at a
+ * call site because there is no call site. */
+el_val_t  el_singleton_acquire(el_val_t id);                    /* §18.1 process identity */
+el_val_t  el_config_declare(el_val_t name, el_val_t type,
+                            el_val_t deflt, el_val_t has_default,
+                            el_val_t required);                 /* §18.2 config schema    */
+el_val_t  el_config_validate(el_val_t program_name);            /* §18.2 startup validate */
+
+/* config(key) — the READ side, and the only one programs write by hand. With a
+ * schema declared it is a validated lookup; without one it degrades to getenv.
+ * (Defined in el_runtime.c but previously never prototyped here, so any program
+ * calling it failed to compile under -Werror=implicit-function-declaration.) */
+el_val_t  config(el_val_t key);
+
 /* Subprocess */
 el_val_t  __exec(el_val_t cmd);
 el_val_t  __exec_bg(el_val_t cmd);