docs: add Runtime Authoring Guide

[JAORMX]

This PR adds a new implementation-agnostic Runtime Authoring Guide.

Key points:

• Defines a stable behavioral contract for new runtimes (capabilities, lifecycle, transports, permissions, isolation, labels, idempotency, error handling, observability, testing, security posture, performance, portability, and acceptance criteria).
• Adds an appendix clarifying the runtime obligations for MCP_TRANSPORT and MCP_PORT (and compatible FASTMCP_PORT/MCP_HOST), including precedence and merge strategy, isolated vs non-isolated behaviors, and separation of internal MCP_PORT vs externally published host port.

Document path:

• docs/runtime-implementation-guide.md

This should help future runtime authors implement consistent behavior across platforms while leaving room for platform-specific optimizations.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 48.14%. Comparing base (8e9766e) to head (1d078b6).
⚠️ Report is 72 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1890      +/-   ##
==========================================
- Coverage   48.17%   48.14%   -0.04%     
==========================================
  Files         233      233              
  Lines       29247    29247              
==========================================
- Hits        14091    14082       -9     
- Misses      14120    14126       +6     
- Partials     1036     1039       +3     

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[coveralls]

coverage: 42.926% (+0.04%) from 42.885%
when pulling e956dff on docs/runtime-authoring-guide
into 736602f on main.

[coveralls]

coverage: 42.882% (-0.003%) from 42.885%
when pulling e956dff on docs/runtime-authoring-guide
into 736602f on main.

[JAORMX]

@yrobla @eleftherias this could use another review.