Four smaller defects in the test-report path: magic-triple indicator, in-place Check mutation, silent depth flattening, last-writer-wins sweep band

[sotashimozono]

Grouped because each is small and they share an area.

1. The 1/0 indicator sentinel is detected heuristically. src/testset.jl:605 excludes a check from _worst_margin iff kind === :abs && tol == 0.5 && want == 1.0. A hand-written @expect got=1.0 want=1.0 tol=0.5 -- legal @benchmark input -- is silently excluded too. The indicator should be a field on Check, not a magic triple.

2. _place_check! mutates a shared Check in place. :1284 assigns chk.id on the caller object. On the direct (non-dump) render path these are the live PinaxTestSet own Checks, so rendering the same tree twice renumbers them from a fresh counter. Harmless today; the fold is not idempotent by construction on any input it does not own.

3. Testset nesting deeper than two levels is flattened without notice. _enter_section! (src/document.jl:587) pushes into pg.sections and _exit_section! (:615) unconditionally clears CTX.section, so a grandchild @testset becomes a sibling of its parent. docs/src/test2pinax.md:53 says "each nested @testset a section" and never mentions a depth limit.

4. _push_sweep_figures! is last-writer-wins on the reference line. src/testset.jl:1000-1005 overwrites wantv[i], bandlo[i], bandhi[i] per legend series inside the for lg in legs loop. With two or more series having different want at the same x tick, only the last series reference and tolerance band is drawn -- for all series.