Add PDF coding standard document

[Copilot]

Addresses issue #2 — creates docs/pdf-coding-standard.md covering the project's conventions and standards.

Document covers:

• Project architecture — layering rules (public API vs internal implementation)
• Naming & class design — Pdf/Excel prefixes, sealed classes, internal constructors, readonly struct with IEquatable<T>
• API patterns — fluent builder, static factory methods, default parameters
• PDF 1.4 spec conventions — object tree structure, content stream operators (Tf, Td, Tj, rg), text positioning reset pattern, string escaping, InvariantCulture numeric formatting
• Color handling — RGB 0–1 model, hex/ARGB parsing, clamping, fallback to black
• Excel integration — zero-dependency xlsx parsing via ZipArchive/XDocument, style→font→color resolution chain, column scaling, pagination
• Error handling — defensive defaults over exceptions, Math.Clamp, graceful fallback for malformed input
• Testing — {Method}_{Scenario}_{Expected} naming, in-memory xlsx generation, temp file cleanup
• Security & dependencies — nullable enabled, zero external NuGet deps in library, allowed test deps

Original prompt

finish

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