docs: Standardize all IndieAuth spec references to W3C URL

- Updated 42 references from indieauth.spec.indieweb.org to www.w3.org/TR/indieauth
- Ensures consistency across all documentation
- Points to the authoritative W3C specification
- No functional changes, documentation update only

Generated with Claude Code

Co-Authored-By: Claude <noreply@anthropic.com>

docs/reports/indieauth-client-discovery-root-cause-analysis.md

@@ -29,7 +29,7 @@ The IndieAuth specification has evolved significantly:
 
 ### 2. Current IndieAuth Specification Requirements
 
-From [indieauth.spec.indieweb.org](https://indieauth.spec.indieweb.org/), Section 4.2:
+From the [W3C IndieAuth Specification](https://www.w3.org/TR/indieauth/), Section 4.2:
 
 > "Clients SHOULD publish a Client Identifier Metadata Document at their client_id URL to provide additional information about the client."
 
@@ -429,7 +429,7 @@ Switch to self-hosted IndieAuth server or different provider
 
 ## Related Documents
 
-- [IndieAuth Specification](https://indieauth.spec.indieweb.org/)
+- [IndieAuth Specification](https://www.w3.org/TR/indieauth/)
 - [OAuth Client ID Metadata Document](https://www.ietf.org/archive/id/draft-parecki-oauth-client-id-metadata-document-00.html)
 - [RFC 3986 - URI Generic Syntax](https://www.rfc-editor.org/rfc/rfc3986)
 - ADR-016: IndieAuth Client Discovery Mechanism