Add bounded primary directory paging

[smiggleworth]

Summary

• add bounded primary-key paging through caller-owned and short read-only transactions
• bind opaque cursors to the route, directory, and primary query shape
• prove historical and current rows page without an index, backfill, or request-path writes
• release the additive API in package version 1.4.1 while retaining assembly version 1.0.0.0

Verification

• dotnet format Fitz.sln --verify-no-changes --no-restore --severity warn
• dotnet build Fitz.sln -c Release --no-restore
• 467 core unit tests
• 16 analyzer tests
• 35 testing-package tests
• 38 extensions tests
• packed package consumer from all five 1.4.1 packages
• fresh osx-arm64 Native AOT packed consumer publish and run

Refs #36

[smiggleworth]

Independent review completed at head 6910a7f65179b7d81c5c1cbba2442f6ccc2ae1ed.

No functional, compatibility, cursor-safety, AOT, or release blockers remain. Review confirmed route- and directory-bound purpose-separated cursors, bounded limit + 1 reads, caller-owned transaction lifecycle, read-only client convenience behavior, and no backfill or write path.

Refinement applied during review: the shared out-of-range cursor message now says "selected query range" so it is accurate for both primary and indexed queries. The README introduction now names both bounded query modes.

Exact-head CI run 35585892102 is green across fast, package/Native AOT, and all four broker matrices.