Never crash: steering rules, clippy.toml, forbid unsafe

- AGENTS.md: a "Never crash" section ahead of the asset rules (the rule, why, the bans,
  Result/? patterns, hostile input-derived numbers, bounded recursion, lock poisoning, the
  last-resort guard, a regression test per crash fix), linking craftrules
  standards/never-crash.md. CLAUDE.md and CONTRIBUTING.md point to it.
- clippy.toml: allow unwrap/expect/panic/indexing in tests, so the restriction lints can be
  denied crate by crate without touching test code.
- [workspace.lints.rust] unsafe_code = "forbid" (was "deny"); there is no unsafe or
  allow(unsafe_code) in the workspace.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Cargo.toml

@@ -116,7 +116,7 @@ cpal = "0.16"
 # filmcraft-isobmff = { path = "../filmcraft/crates/isobmff" }
 
 [workspace.lints.rust]
-unsafe_code = "deny"
+unsafe_code = "forbid"
 
 [workspace.lints.clippy]
 dbg_macro = "warn"