* feat(smartplaylist): add IsMissing and IsPresent operator types
Add two new Expression types for detecting absent/present tags and
roles in smart playlist criteria. Includes JSON marshal/unmarshal
support and Walk visitor registration.
* test(smartplaylist): add JSON marshal/unmarshal tests for isMissing/isPresent
* feat(smartplaylist): add SQL generation for isMissing/isPresent operators
Tags check json_tree(media_file.tags) for key existence.
Roles check json_tree(media_file.participants) for key existence.
Regular DB column fields are rejected with an error.
* test(smartplaylist): add e2e tests for isMissing/isPresent operators
Tests cover tag presence/absence with selective matching (grouping),
universal absence (lyricist role), universal presence (composer role),
and combined operator usage.
* refactor(smartplaylist): use strconv.ParseBool in IsTruthy
Replace hand-rolled string truthiness check with strconv.ParseBool,
which correctly handles standard boolean strings and rejects
unrecognized values as false.
* refactor(smartplaylist): clarify missingExpr parameter naming
Rename defaultNegate to checkAbsence and extract truthy local for
readability. The XNOR logic (checkAbsence == truthy) is now easier
to follow: isMissing passes true, isPresent passes false.
* refactor(smartplaylist): reuse jsonExpr in missingExpr, improve errors
- tagCond/roleCond now handle nil cond (existence-only check)
- missingExpr delegates to jsonExpr(info, nil, negate) instead of
building SQL manually
- Better error messages: unknown fields now report the field name
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