[casoon]

Part of #791 (does not close it).

What

• --sample-strategy first-n|stratified (default first-n, unchanged behaviour). stratified groups batch URLs by URL structure (language prefix, first path section, non-HTML extension) and draws deterministically round-robin, with --max-pages as total budget; picks inside a stratum are evenly spaced, output keeps input order. Needs a batch input and --max-pages.
• sample.selection = "stratified" plus sample.strata[] (key, discovered, selected); schema, OUTPUT_CONTRACT, PDF scope text (de/en) and changelog updated. Still only a sample; failed pages stay in summary.coverage.
• Tests: all strata covered, deterministic, budget respected, within-stratum spread, first-n default unchanged, args validation.

Still open in #791

• --perf-runs N (repeated performance measurement with min/median/max): touches the canonical and throttled performance passes in the pipeline; not done half-way here.
• Selection by page type/template beyond URL structure, and critical process states.
• Fixture-based verification of performance repeats.

Checks: cargo clippy --all-features --all-targets -- -D warnings, cargo test (incl. doctests), cargo check --all-features.