Update architecture.md with libome

[AkshatRai07]

In this PR, we update the architecture.md file with information about the library libome and the branch extern-libome, as mentioned here.

[Copilot]

🟡 Changes recommended

The new documentation includes a factual claim about PR #562 being closed that does not match the current PR state and should be corrected to avoid stale/inaccurate architecture notes.

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Pull request overview

Updates the project architecture documentation to capture the status and constraints around integrating the external C++ libome library for $\mathcal{O}(\alpha_s^3)$ OME computations, including historical context about the extern-libome branch/prototype work.

Changes:

• Add a new “6.5” documentation section describing intended libome integration for as3 OMEs and the current blocking mismatch between $x$-space vs Mellin $N$-space.
• Reference the prototype work associated with PR #562 / extern-libome for future revisit.

File summaries

File	Description
extras/architecture.md	Adds a new architecture section documenting the external libome integration context and constraints.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 1
• Review effort level: Lite

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