Support a load_errors sink in the (relations ...) CSV loading form

[hbarthels]

Summary

Adds an optional trailing (load_errors :name) clause to the generalized (relations ...) CSV loading form, naming the relation that collects rows which fail to load:

(relations
  (keys
    (column "id" INT))
  (relation :wide
    (column "label" STRING)
    (column "ratio" FLOAT))
  (load_errors :my_load_errors))

The load-errors relation has a loader-determined schema that differs from the declared keys and the per-relation value columns. Making it its own clause — rather than another (relation ...) — keeps that difference explicit at the syntax level.

Changes

• Spec: optional RelationId load_errors = 5 on TargetRelations. Presence-tracked, so "no sink" is distinct from a default-valued one. Additive — buf breaking against main is clean.
• Grammar: new load_errors nonterminal, optional trailing element of target_relations. Accepted after both plain targets and CDC insert/delete deltas. No grammar conflicts — the generator's multi-token lookahead distinguishes ("relation from ("load_errors.
• SDKs: regenerated protobuf bindings, parsers, and pretty printers for Python, Julia, and Go.
• Julia equality: extends TargetRelations ==/isequal/hash to cover load_errors (on top of the synthetic_key fix in Include synthetic_key in TargetRelations equality and hashing #271).

Tests

• Two round-trip fixtures — relations_load_errors and relations_load_errors_cdc (the latter also exercising (keys synthetic)) — with .bin, pretty, and pretty-debug snapshots.
• Python parser tests: plain case, CDC case, absence leaving the field unset, and rejection of the clause appearing before the target relations.
• Julia equality tests for the new field.

Full suite green: meta 817, Python 855, Julia 174 test items, Go ok.

Note on diff size

Most of the line count is regenerated parser/printer output across the three SDKs — same shape as #269. The hand-written changes are confined to proto/relationalai/lqp/v1/logic.proto, meta/src/meta/grammar.y, the Julia equality files, and the tests.

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