EXPERIMENT: Int return types as data — and the bug that fell out

PREDICTIONS AND RESULTS
  P1 is_int_call's 35 hardcoded names move to data        TRUE
  P2 is_int_name stays -- it is annotation propagation    TRUE
  P3 the dispatch stays -- it is emission                 TRUE
  P4 codegen shrinks ~40 lines                            TRUE  4507 -> 4469
  P5 the design doc's characterisation is WRONG           TRUE
  P6 the moved data also fixes the bug it exposed         TRUE

P5 CORRECTS THE RECORD. el-language-design.md and geometry-vs-code.md both cite
"== lowering to str_eq unless both operand names are in a hardcoded int-name
set -- a literal list of variable names treated as integers" as the paradigm
defect. It is not one. __int_names is populated from TYPE ANNOTATIONS
(param["type"] == "Int"), which is primitive but legitimate type propagation.
The actual defect was is_int_call: 35 hardcoded builtin return types, the same
shape as the temporal 19.

P6 IS A LIVE CORRECTNESS BUG, PRE-EXISTING, NOW FIXED

    let a = str_len("hello")     // no annotation
    let b = str_len("hi")
    let c = a + b                // -> el_str_concat(a, b) on two integers

Verified identical on the pre-change compiler, so not a regression. It compiled
clean, ran, and printed NOTHING where it should print 7. No error at any layer.

The repair is three lines: an unannotated let takes its type from what the
initialiser returns. The return types were already required for dispatch and
were simply never consulted at the binding site. Moving them into data is what
made the gap visible -- reading the code for eight hours did not.

98/98 native + 2 new, 31/31 integration, fixpoint ok.

lang/tools/check/signatures.rel

@@ -31,3 +31,43 @@ el_duration_sub         returns Duration
 el_duration_scale       returns Duration
 el_duration_div         returns Duration
 ttl_cache_age           returns Duration
+
+# Int-returning builtins. Previously 35 hardcoded names in is_int_call().
+# These decide whether `a + b` is arithmetic or concatenation, so the
+# compiler reads them at emission time -- dispatch, not adjudication.
+
+str_len                     returns Int
+str_index_of                returns Int
+str_to_int                  returns Int
+str_char_code               returns Int
+str_count                   returns Int
+str_count_chars             returns Int
+str_count_bytes             returns Int
+str_count_lines             returns Int
+str_count_words             returns Int
+str_count_letters           returns Int
+str_count_digits            returns Int
+str_last_index_of           returns Int
+str_find_chars              returns Int
+native_list_len             returns Int
+el_list_len                 returns Int
+len                         returns Int
+json_get_int                returns Int
+json_array_len              returns Int
+engram_node_count           returns Int
+engram_edge_count           returns Int
+time_now                    returns Int
+time_now_utc                returns Int
+time_diff                   returns Int
+time_add                    returns Int
+time_from_parts             returns Int
+el_abs                      returns Int
+el_max                      returns Int
+el_min                      returns Int
+float_to_int                returns Int
+unix_timestamp              returns Int
+instant_to_unix_seconds     returns Int
+instant_to_unix_millis      returns Int
+duration_to_seconds         returns Int
+duration_to_millis          returns Int
+duration_to_nanos           returns Int