Add Filecoin protocol support matrix and pubsub compatibility docs

[GautamBytes]

Summary

This is a follow-up to the Filecoin DX work in #1260 and should remain a draft until that PR is merged.

This PR closes Module 2 by documenting Filecoin protocol support and gaps in py-libp2p, adding a machine-readable support matrix artifact, and formalizing the existing read-only pubsub observer as the Filecoin-compatible gossipsub reference example.

What changed

Docs / artifacts

• added docs/filecoin_protocol_support_matrix.rst
• added artifacts/filecoin/protocol_support_matrix.v1.json
• linked the new protocol support page from:
  • docs/index.rst
  • docs/examples.filecoin.rst
  • docs/filecoin_architecture_positioning.rst
• extended docs/filecoin/parity_matrix.md with behavior-level gap rows and a pointer to the new support matrix

Filecoin pubsub example

• kept examples/filecoin/filecoin_pubsub_demo.py as the single Filecoin-style pubsub example
• extended its JSON/config snapshot to report:
  • gossipsub protocol IDs
  • strict signing mode
  • message ID strategy
  • mesh parameters
  • score mode
  • observer-mode limitation
  • current compatibility limitations

Tests

• added tests/core/filecoin/test_protocol_support_matrix.py
• extended tests/core/filecoin/test_examples_filecoin.py
• added a pubsub regression fix/test for bytes-style control-message IDs seen from live peers

Scope / non-goals

This PR does not implement:

• Filecoin hello runtime behavior
• Filecoin chain exchange request/response behavior
• full gossipsub scoring/gater/allowlist parity

Those are documented explicitly as tracked gaps with priorities.

Stacking note

This PR depends on #1260 and should stay as a draft / stacked PR until that branch lands.

[acul71]

@GautamBytes Can you fix the merging issues?