fix(tests): mutation_testing_tests MUT-05/06/07 read ci.yml from the crate dir — dark on every box (#3126)

[noahgift]

Closes #3126.

crates/aprender-core/tests/includes/mut0.rs resolved .github/workflows/ci.yml with a repo-relative Path::new(...), but cargo and nextest run every test binary with cwd = the crate dir (crates/aprender-core), so MUT-05/06/07 could not pass on any box — the target was dark until the quick tier ran it (measured 2026-09-11 15:47Z, cargo nextest run -p aprender-core --tests: 3 failed). Every repo path in the file now anchors on CARGO_MANIFEST_DIR/../.. (the monorepo_invariants.rs pattern; same class as the #3112 fixes for falsification_model_oracle_tests, realizar_integration_tests, toyota_principles_tests), and each panic prints the path it tried.

Written by an agy goal lane (PMAT-1098 phase 3, conversation in the receipt); re-verified by the orchestrator:

cargo nextest run -p aprender-core --test mutation_testing_tests --no-fail-fast
     Summary [   0.025s] 16 tests run: 16 passed, 0 skipped
bash scripts/check_tree_reader_tests.sh   -> exit 0 (registry unchanged)
cargo fmt -p aprender-core -- --check     -> exit 0

No roadmap change on purpose: this PR stays mergeable after the 0.67 train (#3127) squashes.

🤖 Generated with Claude Code

[noahgift]

Auto-merge disarmed until the 0.67 train (#3127, carries the #3112 aarch64 lint fix) lands: gx10 pools carry clean-room now and this branch is off main @ bbd3243 without that fix, so a merge-group ci / lint on gx10 would evict it. Re-arm after gh pr update-branch 3128.

[github-actions]

§13.11 rung 1 — quorum shadow verdict

S13-SHADOW pr=3128 head=04a1c80c74cf86f4d5fee11efbbaaa763005ca5a 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]

Superseded: main already carries this fix — train #3127 (cb829fc) landed mut_workspace_root() for MUT-05/06/07 in crates/aprender-core/tests/includes/mut0.rs via the #3131 sweep; this branch's remaining diff is the same intent expressed with an explicit canonicalize. Closing #3126 with the sha.