dtls13: restore saved flight on resend failure

[zRedShift]

Draft / stack notice

This is a stacked draft, not a standalone PR against main yet. GitHub currently shows the full dependency train in the diff.

Dependency stack currently included in this branch:

1. dtls: defer replay commits until datagram parse succeeds #121 dtls: defer replay commits until datagram parse succeeds
2. dtls13: preserve app data after same-datagram KeyUpdate #122 dtls13: preserve app data after same-datagram KeyUpdate
3. dtls13: preserve overlapping KeyUpdate flights #123 dtls13: preserve overlapping KeyUpdate flights
4. dtls13: bound ACK tracking during handshake replacement #120 dtls13: bound ack tracking during handshake replacement

The actual #124-specific commit is only:

• 35088fe dtls13: restore saved flight on resend failure

So this should be reviewed either after the dependency PRs land, or by comparing only against stack base 17475c7530ad62c5e9c6b87239f1327d2a6c6fca.

Summary

• preserve saved DTLS 1.3 flights when resend or local KeyUpdate output hits transmit backpressure
• keep retry state resumable after partial queued progress
• surface immediate timeout-driven progress when pending post-KeyUpdate ACK/response work remains after output backpressure

Validation for the current stack base

• cargo fmt --check
• extern/scratch/dimpl/scripts/check-snowflake-local.pl 17475c7530ad62c5e9c6b87239f1327d2a6c6fca
• git diff --check 17475c7530ad62c5e9c6b87239f1327d2a6c6fca..
• cargo test --test dtls13 key_update --features rcgen
• cargo test --all-targets --features rcgen
• cargo clippy --all-targets --features rcgen -- -D warnings

CHANGELOG uses the expected PR number #124.

[algesten]

The sheer size of this one seems iffy. It also breaks architecture in multiple ways.