Add multi-scope regression tests for scoped-build implicit-dep detection

[typeless]

Summary

Adds a regression test fixture and four [multi-scope] scenarios for the scoped-build implicit-dependency fix landed in 315b2a2.

The fixture (test/e2e/fixtures/scoped_implicit_dep_multi_scope) sets up three TUs (alpha, beta, main) that share an out-of-scope header at include/lib/shared.h, linked into a single binary at app/app. Mirrors the production iOS bug shape (multi-arg scoped invocation, header outside any scope dir).

Four scenarios in test/unit/test_e2e.cpp:

• Full initial build + scoped rebuild — baseline reproduction
• Out-of-tree variant build (-B build/v1) — variant-aware path
• Scoped initial build + scoped rebuild — probes whether index built under a scoped graph still records the implicit edges needed for the fix's exemption logic
• Full initial + noop scoped pass + scoped rebuild after edit — probes whether intermediate noop scoped invocations leave stale state

All four assert binary_before \!= binary_after on the linked artifact, which catches both the original "Nothing to do (up to date)" failure mode and the "rebuilt some + linked stale" partial-rebuild mode described in pup-header-detection-bug.md's 2026-05-15 follow-up.

RED verification

Reverted the binary_search(implicit_dep_files…) clause from find_changed_files_with_implicit (the one-line fix added in 315b2a2). All four scenarios fail immediately at REQUIRE_FALSE(result.is_noop()) with Nothing to do (up to date). — the exact symptom from the bug doc. Fix restored, all green. The tests genuinely exercise the bug.

Test plan

• make test — full suite passes (461 cases, 23 622 assertions)
• ./build/test/unit/putup_test "[multi-scope]" — 4 cases / 30 assertions
• RED proven: temporary fix revert → all four fail with the documented symptom
• GREEN restored: fix back in place → all four pass
• make format clean

Refs 315b2a2.

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