Align rename references with deterministic resolution

[callumalpass]

Summary

• make filename-based rename fixtures genuinely filename-based instead of accidentally matching same-valued IDs
• require rename reference rewriting to follow the merged same-directory / shortest-path / lexical selector
• retain the separate configured-ID stability fixtures unchanged

Why

The deterministic simple-link contract already selects a winner for duplicate basenames. Rename/reference rewriting must consume that same outcome rather than treating every duplicate basename as ambiguous. Several older filename-rewrite fixtures also assigned IDs identical to basenames, contradicting the configured-ID stability group.

Evidence

• mdbase-rs Phase 1 implementation passes 1,794/1,794 fixtures with this change
• independent adversarial matrices cover same-directory, shortest-path, lexical winners/losers, duplicate IDs, stable IDs, aliases, anchors, embeds, target scoping, order independence, and dry-run parity
• git diff --check

No collection or wire format changes.