GFQL mark(): advanced name features (Phase 2)

[lmeyerov]

GFQL mark(): Advanced name features (Phase 2)

Follow-up to #820 once the core name-first behavior ships. This tracks the remaining enhancements we deferred during the first pass.

Scope

• Implement name_conflicts='suffix' (and any additional policies) so duplicate matcher names can be emitted separately (dup, dup_1, …) in both node and edge outputs.
• Add mode='first' / 'last' options in mark() to expose seed-only or terminal-only markings (requires wavefront bookkeeping in hop/chain).
• Provide helper utilities like validate_mark_names() and list_available_marks() that can lint/discover names before execution (including nested let/ref DAGs).
• Any extra UX polish that builds on the conflict-policy foundation (docs, tooling integrations, etc.).

Prerequisites

• Phase 1 (issue #820) merged with conflict policy + name-first defaults.

References

• Original proposal (pre-split): see history of #820.
• Bug fix groundwork: #817 / #818.