fix(columnar): retain canonical input relation names

[justinjoy]

Summary

• retain session-owned canonical relation names for pending insert/remove evaluation
• use canonical names consistently for retraction relation construction and cache invalidation
• add mirrored Easy/Advanced regressions for mutable caller buffers across snapshot and delta evaluation

Root cause

The session synchronously resolved a relation but stored the caller-provided
relation-name pointer in last_inserted_relation / last_removed_relation.
Reusing or freeing that buffer before step() or snapshot() changed the
later affected-strata lookup and could silently suppress incremental deltas.

The pending state now stores the canonical col_rel_t::name, whose lifetime
is owned by the session.

Validation

• Easy/Advanced/parity and differential/retraction/callback/compound focused tests: 8/8 passed
• ABI suite: 32/32 passed
• original mutable-buffer shared-library reproducer: deltas=1, exit 0 (previously deltas=0, exit 14)
• git diff --check

Closes #931