Define source-faithful Hoster-Stump interlacing sequences

[PerAlexandersson]

Summary

• define the Section 2 zero/nonzero-splitting convention and source-specific pair relation
• include the degree-at-most-one exception omitted by local Prec
• define a nonnegative, elementwise-real-rooted, pairwise source interlacing sequence
• preserve all four exact Lemma 2.3 list formulas and document the printed part (4) endpoint typo
• prove checked counterexamples for all four false weak IsInterlacingSeq0Nonneg closure interfaces

Validation

RealRooted.HosterStumpInterlacing built successfully from current origin/main (8568/8568 jobs).

Fixes #326.

[PerAlexandersson]

Follow-up source audit: the source-exact predicate in this PR is faithful, but Hoster--Stump Lemma 2.3(2) is false under the paper's unconditional degree-at-most-one convention. Stacked PR #332 adds an exact checked counterexample and documents why issue #316 needs a stronger pairwise Prec0 hypothesis. No definition in #331 is changed by the stacked commit.