Fix signature header boundaries and exponent rendering - #9531

Summary

Fix two independent string-processing defects in the shared text-signature machinery:

• Stop internal signature-header parsing at the first blank paragraph, matching CPython skip_signature. Documentation stripping uses the same boundary check, so malformed headers retain their original docs.
• Preserve every decimal exponent digit in typed float defaults instead of treating a three-digit exponent as a two-digit one. Reuse the existing integer renderer and retain minimum two-digit padding.

The commits add pure string/formatting unit tests only. They do not construct a VM, change argument binding, alter method-flag semantics, or change Lib tests.

Evidence and scope

The header contract is grounded in CPython 3.15 Objects/typeobject.c.

The exponent bug follows directly from emitting b'0' + exp / 10 as one byte when exp >= 100. Tests cover signed and unsigned one-, two-, and three-digit exponent boundaries and the sizing pass.

These are generic API defects discovered during the whats_left investigation, not yet identified as causes of a specific reported mismatch. This PR is independent of the report-only #9530. It does not implement CPython's method-flag signature fallback or claim full shortest-round-trip float rendering; the existing inaccurate comment about that guarantee is corrected.

Validation

• Independent source review, normal commit hooks, and git diff --check: passed.
• Local compile/unit/runtime: not run; dependency-cache/DNS/escalation bootstrap blocker remains.
• Existing CI, including the added pure unit tests and all normal compatibility suites, is pending on this exact head. Draft until that evidence is available.