feelc structured error schema

Compilation errors (parsing .rules + typecheck/lowering) are structured, positioned objects, serializable to JSON via --json. This is the contract that the authoring skill reads for its red→green loop (fixing the source from line/col/suggestion).

Text format (always)

Error() remains backward-compatible:

without a known file: line <N>: <message>
with a file : <file>:<line>[:<col>]: <message>
global error (without position): <message>

The suggestion never appears in the text (only in JSON / a dedicated human rendering) so as not to break assertions on substrings.

JSON format (`--json`)

Emitted on stdout by feelc run|verify|check --json when compilation fails:

{
  "file": "credit.rules",
  "line": 12,
  "col": 7,
  "code": "DSL002",
  "message": "invalid FEEL cell \"1 +\": ...",
  "suggestion": "..."
}

Field	Type	Presence
`file`	string	omitted if unknown
`line`	int	always (0 if position unknown)
`col`	int	omitted if 0 (unknown); 1-based
`code`	string	omitted if empty; stable (see below)
`message`	string	always; identical to the text
`suggestion`	string	omitted if empty

col is computed at the DSL split (offset of the cell segment within the source line). It is reliable for table cells (conditions/outputs); for literal-expression expressions and single-line declarations, only line is guaranteed.

Code catalog (STABLE — do not renumber)

Consumed by the skill: these codes are a contract. Sources: internal/diag/diag.go.

`DSL*` — `.rules` parser

Code	Meaning
DSL001	unrecognized instruction
DSL002	invalid FEEL cell / expression (wraps the FEEL cause)
DSL003	model without a `model "..."` declaration
DSL004	malformed `model` header
DSL005	malformed `input`
DSL006	malformed decision header
DSL007	unrecognized decision body line
DSL008	malformed rule (`=>` missing)
DSL009	empty cell
DSL010	malformed `type` declaration
DSL011	unsupported type
DSL012	content after `{` on the header line

`CMP*` — compiler / typecheck

Code	Meaning
CMP001	reference to an undeclared name (`needs`/var)
CMP002	unsupported hit policy
CMP003	unknown decision type
CMP004	wrong number of conditions / outputs
CMP005	PRIORITY constraint not satisfied
CMP006	COLLECT constraint not satisfied
CMP007	construct outside the v2 subset
CMP008	literal expected

Scope

This schema covers compilation errors. The reports from verify (Finding) and check (Verdict) have their own JSON form (aligned with the same style), unchanged here. The raw FEEL cause remains wrapped (Unwrap() / errors.As), never rewritten.