XL-0-CARRIER: root-cause the Rc/Node/Witness carrier-shape survivors as one class before repairing three sites

[briansrls]

Auto-opened by session-dashboard for session gentle-koi-869.
Pushing to session/gentle-koi-869-carrier advances this PR.

Worker attestation

Before flipping this PR to ready for review, confirm each item:

• Title describes the change (not the session id or branch).
• PR body summarises what and why (replace the TODO below).
• Tests run: name the command (e.g. npm test, cargo test) and the result.
• If this closes a work item, the body contains a Closes #N directive.
• No commits on this branch are surprises (no fork/cherry-pick I did not make).
• No secrets / credentials / large binaries staged.

Summary

TODO: replace this paragraph with one or two sentences naming the change and its motivation. Reviewers read this first.

Test plan

• TODO: list the commands that ran (or "no tests changed; relied on CI") and the outcome.

[gunbai-bot]

Closing: this PR contains no unlanded work, and merging it would be a large revert.

It was auto-opened by session-dashboard on session/gentle-koi-869-carrier after #9855 squash-merged. A squash leaves the branch permanently "ahead" of main by commits whose content is already in main, so the dashboard reopens it.

Verified rather than assumed:

• XL-0-CARRIER: two emitter decisions that fabricate where they cannot answer, and the projection a sealed carrier owed #9855 merged as 5998d60edd; every symbol this branch introduced is present in origin/main at the same occurrence count — rust_param_type_is_type_variable (5), rust_witness_carrier_from_witness_headed_type_node (6), WitnessCtorPathUnresolved (6), rust_witness_ctor_refusal_expr (4), and both enrolled tests, generic_parameter_declines_the_fail_closed_unwrap (3) and witness_carrier_declines_a_non_witness_expected_type (3).
• src/v2/compiler/source_authority.dag is byte-identical between this branch and main.
• The three-dot diff GitHub renders is against the pre-squash merge base, so it shows this branch's changes as additions. The real two-dot diff origin/main..HEAD is 125 files, +3,807 / −12,683 — this branch is now far BEHIND main, and those deletions are other lanes' landed work, not anything of mine.

So flipping this to ready would propose reverting ~12.7k lines from a branch whose own contribution is already merged. Nothing to salvage here; the work is in main.

— sent from gentle-koi-869