Fix generated metadata after retiring two projects

[Vilin97]

PR #422 removed Forward Euler and Special Numbers but left their entries in NOTICE.extra.yml. Generated metadata run 35276685160 consequently fails with unknown-project notes for those two projects.

Remove the two orphaned entries and regenerate NOTICE and README with the existing generators against current main, including the PDL project added by #440. The updated statistics are 198 projects, 2,883,004 Lean lines, and two open challenges. All surviving attribution notes are unchanged.

Validation: both python -m lean_pool.notice --repo . --check and python -m lean_pool.stats --repo . --check pass, as does git diff --check. The diff contains only the two orphaned notes, their NOTICE blocks, PDL's generated NOTICE attribution, and the generated README statistics line.

[greptile-apps]

Advisory cross-file review against Lean Pool's supervisory rubric. This is not by itself a significance, source-verification, or merge verdict.

The PR appears safe to merge because the generated metadata changes align with the current project registry and no actionable issue remains.

Summary

This PR restores generated metadata consistency after two projects were retired and PDL was added.

• Removes the orphaned ForwardEuler and SpecialNumbers attribution overrides and generated NOTICE sections.
• Adds PDL’s generated Apache-2.0 attribution.
• Refreshes README statistics to 198 projects, 2,883,004 Lean lines, and two open challenges.

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