Add TIP-935: Harden ECDSA Signature Validation (Last Call) - #939

Summary

Add TIP-935, Harden ECDSA Signature Validation, from the specification in #935, and list it in README.md in numerical order. The TIP enters Last Call for wider review, retaining its original author and creation date.

The specification preserves the inclusive 65–68-byte signature-length range at RPC/P2P transaction admission, auxiliary queries, and relay HELLO, and in strict transaction and direct witness validation after governance activation. Recovery interprets the first 65 bytes while accepted transaction signatures retain their full original bytes. P2P witness normalization separately retains a copied 65-byte prefix.

At P2P transaction ingress, any signature outside 65–68 bytes rejects the entire batch and disconnects the sender with BAD_TX, before task submission or request cleanup. Existing disconnect recovery handles outstanding requests. RPC length failures return SIGERROR.

The TIP also covers strict recovery checks, historical compatibility, permission migration, and required conformance coverage. It reflects the ECDSA-only release, auxiliary-query error precedence, Java API migration, and temporary activation-time capacity impact.

Discussion: #935.

Test plan

• Check the updated P2P admission rules against the implementation.
• Confirm the TIP and issue have matching content apart from the existing Draft/Last Call wording, metadata-fence formatting, and CC0 link differences.
• Validate metadata, retained sections, title length, date format, and README ordering and status.
• Run git diff --check on the documentation update.

This PR adds documentation only. The implementation's regression tests were inspected, not executed for this documentation update; the conformance and integration tests listed in the TIP remain implementation requirements.