Restore TextStrategy::Tokenize with vault integration

[martsokha]

Context

TextStrategy::Tokenize was removed in alongside Pseudonymize because the existing implementation was a placeholder — it routed through the same string-substitution path as Replace, providing no actual token vault.

What's needed

Re-introduce TextStrategy::Tokenize { vault_id } (and TabularStrategy::Tokenize if scope warrants) backed by:

• Vault integration: pluggable backend (in-memory for dev; external vault for prod) keyed by vault_id.
• Token format: opaque, non-guessable (UUID, ULID, or vault-specific).
• Reversibility: vault round-trip — given a token + vault_id, return the original value (with auth).
• leak_profile(): LeakProfile::Recoverable.

Acceptance

• TextStrategy::Tokenize { vault_id: "ssn-vault" } produces opaque tokens.
• Same original always produces the same token within a vault.
• Round-trip test: tokenise → look up → recover original.
• Vault config wired through RuntimeConfig so operators can swap backends.

Why removed

Same as #241 (Pseudonymize) — placeholder routing was misleading. Removed to keep the strategy surface honest until a real vault integration lands.