Re-runnable domain scripts: exec --continue-on-error + idempotent ADD/DROP ATTRIBUTE

[ako]

Sudoku-app finding #10: domain-delta scripts couldn't be replayed. alter entity … add attribute X errored attribute 'X' already exists on a second run, and because exec halts on the first error, a script that was 90% already-applied applied none of the remaining 10%. This app needed 25 single-statement alters where 3 would have done, and couldn't re-run 01/02 at all.

Two complementary mechanisms, one per commit:

mxcli exec --continue-on-error (general)

Attempts every statement instead of halting at the first failure; each failure is reported as statement N: … and the command exits non-zero if any failed. A real error is still surfaced — never masked — and exit/quit still stop the run. Works for every statement type, so any partially-applied script becomes re-runnable.

IF NOT EXISTS / IF EXISTS on attributes (surgical)

ALTER ENTITY … ADD ATTRIBUTE IF NOT EXISTS X: T and … DROP ATTRIBUTE IF EXISTS X turn the already-applied case into a skip-with-notice instead of an error, so a domain script re-runs cleanly and converges. The plain forms still error, so the guard is explicit and opt-in. (Grammar → AST → visitor → executor, full pipeline.)

alter entity Sudoku.Game add attribute if not exists PuzzleNo: integer default 1;
alter entity Sudoku.Game drop attribute if exists LegacyFlag;

Tests & docs

• ExecuteProgramContinueOnError unit tests (mixed success/fail counting; exit still halts).
• Visitor test (guards set the AST flags; plain forms don't) + executor mock tests (add-existing → skip, add-new → applied, drop-absent → skip, plain forms still error).
• Bug-test example, mxcli syntax entry, MDL_QUICK_REFERENCE.md rows, and a fix-issue.md symptom row.

Generated ANTLR parser files under mdl/grammar/parser/ are not committed (regenerated by make).

🤖 Generated with Claude Code

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Generated by Claude Code