feat(ci): add regression CI gate and WASM size enforcement (#424, #425)

[mctituschristian]

Issue #424 — Regression Test Suite CI Gate for Slice Failover

• Add 8 regression tests to slice_failover_tests.rs covering previously fixed bugs: threshold=1 immediate activation, failure count after failover, multiple-backup priority order, revert-then-refail cycle, unknown slice count=0, failure with no backup, event emission counts, and unregistered config returning false
• Annotate the 'Run tests' CI step as the required branch-protection check for regression_tests.rs and slice_failover_tests.rs
• Document regression-test policy in docs/best-practices.md (when to add a regression test, naming conventions, CI gate, merge checklist)

Issue #425 — WASM Size Budget Enforcement in CI

• Add 'Build WASM artifact' step: cargo build --target wasm32-unknown-unknown
• Add 'Check WASM size budget' step: fails build when ttl_vault.wasm exceeds 512 KB hard limit; prints warning when between 460 KB and 512 KB
• Update docs/wasm-size-budget.md to document both thresholds, enforcement policy, and example CI output for all three size states

Closes #424
Closes #425

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