fix: harden Claro diagnostics and runtime guards

Preserve source locations, validate malformed JSON, and add opt-in resource limits. Include executable error-guide coverage, learner pilot materials, native Windows acceptance helpers, and updated CI/documentation.

docs/ERRORS.md

@@ -1,29 +1,144 @@
-# Claro Errors
+# Errors and small repairs
 
-Claro tries to report mistakes in beginner-friendly language.
+An error starts with `filename:line:`. Open that file and find that line.
+The line belongs to the function or imported file where the error happened,
+not automatically to the caller. Caught errors expose the same origin through
+`LASTERRORFILE` and `LASTERRORLINE`.
 
-Examples:
+Use `claro check my_program.claro` for command/block mistakes; running the
+program also detects runtime expression/value errors. A checker is not a sandbox.
+The examples below are executed by `tools/validate_error_guide.py`.
+
+## Quote
+
+Save as `quote.claro`. Run `claro quote.claro`.
+
+```claro
+SAY "Start"
+SAY "unfinished
+```
+
+Message:
 
 ```text
-I do not know the command "PRNT". Did you mean "SAY"?
+quote.claro:2: Text is missing a closing quote. Add a double quote at the end.
 ```
 
+One repair:
+
+```claro
+SAY "Start"
+SAY "finished"
+```
+
+## Operand
+
+Save as `operand.claro`. Run `claro operand.claro`.
+
+```claro
+SET score 1
+SAY score +
+```
+
+Message:
+
 ```text
-SET needs TO.
-Try:
-SET name TO "Alex"
+operand.claro:2: This expression needs a value after the operator.
 ```
 
+One repair:
+
+```claro
+SET score 1
+SAY score + 1
+```
+
+## Command
+
+Save as `command.claro`. Run `claro check command.claro`.
+
+```claro
+PRNT "Hello"
+```
+
+Message:
+
 ```text
-IF block opened here is missing ENDIF
+command.claro:1: I do not know the command "PRNT". Did you mean "SAY"?
 ```
 
+One repair:
+
+```claro
+SAY "Hello"
+```
+
+## Assignment
+
+Save as `assignment.claro`. Run `claro check assignment.claro`.
+
+```claro
+SET name
+```
+
+Message:
+
 ```text
-I do not know the variable "socre" yet. Did you mean "score"?
+assignment.claro:1: SET needs a name and a value. Try: SET name "Alex"
 ```
 
-For safer learning, run this before running a new program:
+One repair:
 
-```bash
-./claro check my_program.claro
+```claro
+SET name "Alex"
+SAY name
 ```
+
+## Choice
+
+Save as `choice.claro`. Run `claro check choice.claro`.
+
+```claro
+IF YES
+    SAY "Yes"
+```
+
+Message:
+
+```text
+choice.claro:1: IF block opened here needs END
+```
+
+One repair:
+
+```claro
+IF YES
+    SAY "Yes"
+END
+```
+
+## Variable
+
+Save as `variable.claro`. Run `claro variable.claro`.
+
+```claro
+SET score 10
+SAY socre
+```
+
+Message:
+
+```text
+variable.claro:2: I do not know the variable "socre" yet.
+```
+
+One repair:
+
+```claro
+SET score 10
+SAY score
+```
+
+These teach short `SET` and `END`. Older forms remain supported in
+[compatibility syntax](COMPATIBILITY.md), not required as repairs.
+For a caught error, read its metadata before another error changes it.