GFQL mark(): name-first defaults (Phase 1)

[lmeyerov]

GFQL mark(): Name-First Defaults (Phase 1)

This ticket tracks the first wave of the mark() redesign—the portions we can ship alongside the matcher name conflict fix (#818).

Goals (Wave 1)

• Adopt a name_conflicts policy (any, error) in GFQL execution so duplicate matcher names either OR together (default) or raise a descriptive schema error before execution.
• Teach mark() to return named matchers automatically when present (mode='auto'), and to fall back to existing mark_nodes/mark_edges when no names/projecting is specified.
• Keep project= filtering the marked columns (nodes/edges) after the mode runs.
• Expose basic helper plumbing so the policy travels through gfql(), chain(), gfql_remote(), and DAG (let) execution.
• Document the new defaults and update CHANGELOG/tests accordingly.

Out of Scope (Wave 2 / follow-up)

• name_conflicts='suffix' or other advanced policies (requires deterministic suffixing and additional bookkeeping).
• mode='first' / 'last' in mark() (needs explicit wavefront tracking in hop/chain).
• Static helpers like validate_mark_names() / list_available_marks() for complex DAGs (may require deeper AST analysis).
• Any additional UX polish discovered during implementation (file new tickets as needed).

Wave 2 is tracked separately in #823 so we can land the core behavior change promptly and iterate on richer features afterward.

References

• Bug fix groundwork: #817 / #818
• Phase 2 follow-up: #823