Download libsrt 1.5.7

## Security Notice

This release includes important security hardening and multiple vulnerability fixes identified during extensive security audits of the SRT codebase. Several issues could allow memory corruption, protocol state manipulation, resource exhaustion, or misuse of auxiliary tools and CI infrastructure.

Users are strongly encouraged to upgrade to this version to benefit from these security improvements and protocol hardening measures.

## Security Improvements

### Handshake and Encryption Security

• Fully remediated the KMREQ processing vulnerability by validating all incoming KM message lengths before they reach internal conversion and copy routines, protecting both HSv4 and HSv5 negotiation paths.
• Completed the remediation of the encryption downgrade vulnerability by preventing post-establishment KMRSP messages from modifying the security state of already secured sessions. Additional protections were added for both HSv4 and HSv5 negotiation paths.
• Added minimum MSS enforcement during connection negotiation to prevent undersized payload buffers that could otherwise lead to heap corruption and information disclosure during handshake generation.
• Hardened handshake state processing to correctly derive connection state from the live connection status and prevent unintended state rollback caused by late or malformed handshake exchanges.

### Data Plane Protection

• Fixed validation of ACK control messages to prevent send-buffer corruption caused by forged or malformed acknowledgements. Additional bounds checking now ensures that sequence number ranges remain valid before buffer state updates occur.
• Added protection against invalid DROPREQ ranges. Reversed ranges and invalid sequence number distances are now rejected before modifying receiver buffer state.
• Corrected receive-path connection status handling to prevent non-addressed packets from affecting unrelated connection attempts.

### FEC Robustness

• Added payload-size validation in FEC clipping operations to prevent out-of-bounds writes when processing oversized payloads.
• Introduced minimum-size validation for FEC control packets, eliminating integer-underflow conditions that could occur when processing malformed packets.
• Added upper bounds for peer-supplied FEC configuration values and improved error handling to prevent excessive memory allocation during connection establishment.

### Bonding Reliability

• Fixed a use-after-free condition in the bonding BACKUP send path. Internal member context tracking now safely handles members removed while locks are temporarily released, preventing dangling references during failover processing.

### Application Hardening

• Added validation of remotely supplied filenames in the `srt-file-transmit` utility. Path separators, parent directory references, and platform-specific path manipulation patterns are now rejected before files are created.

## Build and CI Security Enhancements

• Replaced the dynamic Codecov script download mechanism with a pinned and integrity-verified version.
• Pinned ABI compatibility checker dependencies to specific versions and removed reliance on mutable default branches.
• Improved GitHub workflow supply-chain protection by pinning third-party actions, container images, and external dependencies to known revisions.

## Stability Improvements

• Fixed local connection teardown handling following rejected late handshakes, ensuring the local endpoint correctly terminates invalid connection states.
• Improved error handling for FEC initialization failures and memory allocation exceptions, providing graceful connection rejection instead of abrupt failures.

## Test Coverage

Additional negative and security-focused test coverage has been added for:

• Malformed KMRSP messages.
• Encryption downgrade scenarios.
• ACK validation.
• DROPREQ malformed and reversed ranges.
• FEC oversized payloads and invalid configurations.
• Connection cleanup and shutdown paths.