[noahgift]

ci: guard-cargo#m5 mounts the pinned toolchain, fails loud on ENV (PMAT-6041)

ci/m5-container-rustc-6041 @7a48a6496d (w5382), one commit, linear on main 45aa5d1. Gate-class (CI file): DRAFT and unarmed until seat 2 + quorum + arbiter sign.

#6031 x86-main (run 39875925063) went red on m5: empty "container rustc:" and "could not write settings file /home/runner/.rustup/settings.toml: Read-only file system", so rustc in the container resolved to the rustup proxy.

• ci/sections.yml (~line 1969): m5 resolves the toolchain with readlink -f, mounts it read-only when it lives outside RUSTUP_HOME, and the container step exits FAIL(ENV) naming the resolved rustc when it is not the pinned $TC/bin/rustc.
• docs/roadmaps/entries/PMAT-6041.yaml: ticket entry (epic 5510).
• docs/roadmaps/roadmap.yaml: regenerated roadmap.

Checks run by the author: pre-push four green, fat_driver self-test 60/60, roadmap check ok. Caveat: the runner-side cause (pinned toolchain not visible in the container) is a hypothesis the author could not verify; in the local container probe the assertion passes. If wrong, the new FAIL(ENV) line names the real path on the next run.

Refs #6041

keep-open: #6041 stays open until the m5 fix is seen green on a runner.

ont-delta: none no new type, shape or reason

🤖 Generated with Claude Code

[noahgift]

Seat 2 (w5018, non-author): APPROVE @7a48a6496d0cacd716b9c2d265c46d7cd3c96771

[noahgift]

Seat 1 (aprender-w5404, non-author): APPROVE @7a48a6496d0cacd716b9c2d265c46d7cd3c96771 — gate-class (ci/sections.yml guard-cargo#m5). Read the full diff: tc is resolved with readlink -f; a toolchain outside RUSTUP_HOME gets its own read-only mount (case table checked: under ru -> no extra mount, outside -> -v tc:tc:ro; empty array is safe under set -u via the ${tcm[@]+...} idiom); the container now fails with FAIL(ENV) if rustc does not resolve to the pinned $TC/bin/rustc, so the empty 'container rustc:' symptom from run 39875925063 becomes a loud named failure instead of a silent proxy fallthrough. sections.yml parses (15 jobs). EMG-7 GREEN (1 commit, one PMAT-Ticket, 0 merges); roadmap.yaml +19 matches the PMAT-6041 entry; privacy scan clean. Not measured: a real m5 run on the runner (the fix is only proven when the m5 step goes green), clippy/hooks. Needs a second non-author seat, planted quorum and arbiter sign (gate-class).

[github-actions]

§13.11 rung 1 — quorum shadow verdict

S13-SHADOW pr=6045 head=7a48a6496d0cacd716b9c2d265c46d7cd3c96771 verdict=REFUSE class=Q1 arm_rc=1

Shadow mode: this records a verdict and merges nothing. A refusal
to arm is not a block (§13 adds zero rows to §7) — the pull request is
exactly as green as it was.

[noahgift]

Snapshot before close: head 7a48a64; last run id/name/status: 39885595256 CI completed/failure. Superseded by #6063 (cop order, aprender-57; arbiter OK). Branch kept.