fix(net): normalize two-digit cookie years

[xrip]

Summary

• Normalize legacy two-digit cookie expiry years at the 69/70 boundary.
• Reject normalized cookie years below 1601.
• Add regression coverage for years 00, 69, 70, and 99.

Fixes #616

Validation

• The focused regression failed on unchanged origin/main: year 00 produced Ok(18817999) instead of the year-2000 value Ok(965502799).
• cargo nextest run --release -p obscura-net: 79/79 passed.
• cargo nextest run --release --no-default-features -p obscura-net: 79/79 passed.
• cargo nextest run --release --features render --no-fail-fast: 1397/1398 passed. The only failure is the known Windows refusal case in Windows connection refusal resets before the 503 response is read #604, fixed by fix(cdp): deliver connection refusal response #605.
• Release builds passed for render, render,stealth, no default features, and no default features with stealth.
• Obstacle course: 33/33 with the benchmark input normalization disclosed at fix(obstacle-course): reobserve intersection sentinel obscura-benchmark#3 (comment).
• A Git three-way merge check with the branch for fix(net): delete cookies on expired updates #609 completed without conflict.