Narrow what gets deferred to Pass 2: inline m2m/o2m to already-existing records

[bosd]

Follow-up from the code-review discussion around #10 (xmlid-collision preflight, PR #224).

Idea

Today, relational fields (many2many / one2many) are always handled in Pass 2 (after all records are created). But a relation only needs deferral when it points at other rows created in the same import (forward references). A relation pointing at records that already exist in Odoo (e.g. existing res.partner.category tags, an existing country_id) can be written inline in the Pass-1 load() — no deferral, no second write, and crucially no external id required on the owning row.

Why it matters

• Eliminates a class of the "blank-id row carries relational data" problem feat(#10): pre-flight check for external-id (xmlid) collisions #224 warns about: such rows can carry relations as long as those relations reference existing records — if we stop deferring them.
• Fewer RPCs (one load() instead of Pass 1 + a Pass-2 write pass) for the common "link to existing masters" case.
• Simpler, more predictable behaviour.

Sketch

• During strategy planning (preflight _plan_deferrals_and_strategies / _handle_m2m_field), classify each relational column:
  • references same-import rows (the column's values appear in this file's id column, or it's a self-reference) → keep deferring to Pass 2 (needs the ids created first);
  • references pre-existing records only → keep it in the Pass-1 payload (inline), skip the Pass-2 strategy for it.
• Requires care for m2m/o2m value formats that load() accepts inline vs. what Pass 2 builds today, and for the (6,0) replace semantics (single-source-of-truth default we're keeping — see the 🌱 Initial Commit - Alpha Release Candidate #14 discussion).

Not urgent

This is an optimization + ergonomics improvement, not a correctness bug (the deferred path works). Filing so it isn't lost.

Refs: #224 (collision preflight), and the #14 additive-relations discussion.