release: prepare 2.0.0

[ikelaiah]

Summary

• materialise the frozen mathlib-fp 2.0.0 release candidate
• promote the already-qualified 1.10.0 API without introducing new numerical or public API changes
• add 2.0.0 versioned API evidence, migration guidance, release-candidate metadata, and release-state validation
• keep 1.10.0 as the latest published stable release until final 2.0.0 publication

Compatibility

• no new public API
• no src/*.pas changes
• no numerical algorithm changes
• no tolerance/default changes
• no warning or package-surface changes
• no breaking changes for supported 1.x source
• historical release evidence remains unchanged

Validation

Local validation passed, including:

• git diff --check
• all tools/test_*.py
• API snapshot generation twice with clean second generation
• documentation checks
• API decision checks
• 2.0 promotion checks
• convergence checks
• documentation build and offline archive validation
• full qualify_release.py --release 2.0.0
• normal, optimized, runtime-checked and heap-traced FPCUnit runs
• examples/output contracts
• migration rehearsal
• workflow qualification
• portability/numerical/benchmark gates
• Lazarus package build

RC status

This PR prepares RC1 only.

Do not merge until RC qualification and soak are complete.