fix(guard): resolve_base refused every STACKED merge-group entry — two of every three queue builds were RED by construction (#3186)

[noahgift]

GitHub builds queue entry N as a squash on top of entry N-1's squash, so at
position >= 2 the head has ONE parent that is not the origin/main tip and, at
fetch-depth 1, is not fetched. Every rule in resolve_base refused that shape
(exit 2 -> guard-tree FAIL), so with max_entries_to_build=3 the second and
third entries of every stacked build were RED on this resolver alone:
runs 34704287677 (#3139) and 34704288441 (#3046), 2026-09-12, each after
passing 82,085 workspace tests on gx10. Four PRs were dequeued today on it.

Under GITHUB_EVENT_NAME=merge_group the resolver now deepens the shallow
checkout by one commit (git fetch --deepen=1 origin, else fetch the parent by
sha) and names that parent as the base — exactly this entry's own diff. A
push-shape depth-1 head stays refused (never the tree against itself);
ROADMAP_DIFF_NO_DEEPEN=1 is the mutation.

Case table (check_roadmap_diff_additive.sh --self-test): rows 19-20 added —
a self-contained repo c1 -> c2 -> c3 with origin/main := c1 fetched by name
into a depth-1 clone (a shallow clone cannot update-ref to an object it
lacks; the fixture fetches the tip the way the job does). Row 19 resolves to
c2 under merge_group; row 20 is refused by name with deepening disabled.
Mutation measured: with origin/main's resolver restored, row 19 goes RED.
20/20 rows; bashrs 0 errors on both files (guard: 0 -> 0, lib: 0 -> 0).

Every consumer of the lib benefits: check_hardcoded_paths.sh (its ratchet
also read "origin/main tip" as the base for stacked entries),
check_no_fabricated_baselines.sh, check_dogfood_coverage.sh,
check_row_pr_write_set.sh.

Refs #3186, #3177, APR-RELEASE-001 §3.4/§5 P0·Pack (packing rule, operator
2026-09-12).

Closes #3186.

🤖 Generated with Claude Code

[github-actions]

§13.11 rung 1 — quorum shadow verdict

S13-SHADOW pr=3187 head=ec465060eea25ccd9a06ba843fb468607330660a 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.