Adopt GitHub-flavoured Markdown footnotes and fix numbering/order

[coderabbitai]

Description:
Replace ad-hoc numeric markers in the body with proper GitHub-flavoured Markdown footnote references and ensure the footnote list is ordered by first appearance.

Location: docs/lag-complexity-function-design.md, lines 1496-1752

Current issue:

• Using bare numeric markers (e.g., 1, 3, 4) instead of proper footnote syntax
• Footnote definitions not ordered by first reference
• Inconsistent "accessed on" phrasing and dates
• Current usage breaks the repository's Markdown guidelines

Required changes:

1. Rewrite in-text numeric markers (e.g., bare 1, 3, 4) to [^1], [^2], etc.
2. Renumber the footnote definitions in ascending order of first reference
3. Normalise "accessed on" phrasing and dates

References:

• PR: Add core API scaffolding #1
• Comment: Add core API scaffolding #1 (comment)
• Adopt GitHub-flavoured Markdown footnotes and fix numbering/order #6