docs: cross-repo proposal updates + tier1 production-readiness notes

Two planning documents covering the rationale for everything else in
this branch.

cbm-cross-repo-proposal.md:
* Updated Tier 1 sections to reflect the producer-side detection that
  ships in this branch (ctor params, factories, DI registry, fields).
* New §5.7 Tier 1g — Contract-package FQN extraction. Explains why the
  whole 1g family is deferred to a focused follow-on PR rather than
  landed here, and sequences the work into four pieces (producer dual
  emission, AST-time package extraction, deterministic collision
  resolution, NuGet/Maven consumer-side cache scan).
* Updated §5.8 sub-tier sequencing accordingly.

tier1-extractor-fixes.md (new):
* Documents the four production-readiness gaps that prevented Tier 1's
  producer-side detection from firing on real .NET fleets:
  parallel-pipeline wiring, C# 12 primary-ctor extraction, protobuf
  rpc → service fallback, and graph-UI cross-galaxy passthrough.
* Adversarial-review follow-ups: incremental-pipeline wiring,
  safer project-wide stub-var fallback, and the cross-package
  collision warning + service_qn property mitigation.

Author: sponger94