A sign and an unknown escape are both the reader's business

Two mutations the reader survived: dropping '+' from the number
dispatch, so +5 reads as a symbol nobody defined, and accepting an
unknown string escape as the character after the backslash, so a typo
silently reads a different string. Both now have a row, and the known
escapes are asserted on the decoded bytes rather than through
Form.to_string, which escapes them again and would compare the source
with itself.

test/test_flan.ml

@@ -47,6 +47,15 @@ let () =
   (* ── Atoms ─────────────────────────────────────────────────────── *)
   reads "integer"        "42" "42";
   reads "negative"       "-1" "-1";
+  (* A sign is part of the number, both ways. [+5] is the one that reads as a
+     symbol the moment the '+' case is dropped from the dispatch, and a symbol
+     named "+5" is an unknown name much later and somewhere else. *)
+  reads "leading plus"   "+5" "5";
+  reads "plus float"     "+0.5" "0.5";
+  reads "plus in a call" "(f +5 -5)" "(f 5 -5)";
+  (* And the operator is still itself: [+] alone is the addition symbol, and
+     [(+ 1 2)] must not read its head as a number. *)
+  reads "bare plus"      "+" "+";
   reads "float"          "0.05" "0.05";
   reads "hex"            "0xE6B800FF" "3870818559";
   reads "string"         "\"SAND\"" "\"SAND\"";
@@ -141,6 +150,18 @@ let () =
   rejects "unterminated str"  "\"abc";
   rejects "empty keyword"     ":";
   rejects "unknown char"      "\\bogus";
+  (* An escape the reader does not know is a typo, not a character: accepting
+     \q as 'q' silently reads a different string than the one that was
+     written, and nothing downstream can tell. *)
+  rejects "unknown string escape" "\"a\\qb\"" ~needle:"unknown string escape";
+  (* The escapes it does know still decode, which is what says the rejection
+     above rejects the unknown one and not escaping itself. Asserted on the
+     string's bytes rather than through [Form.to_string], which escapes them
+     again and would compare the source with itself. *)
+  (match Reader.read_all ~file:"<test>" "\"a\\nb\\tc\\\\d\\\"e\\0f\"" with
+   | [ { Form.v = Form.Str s; _ } ] ->
+     check "known escapes" (s = "a\nb\tc\\d\"e\000f")
+   | _ -> check "known escapes: one string" false);
   rejects "metadata"          "^:async";
   rejects "dangling quote"    "'";
   (* Each of these asserts the reason, not merely that something failed. *)