Extract secret detection into secret_redaction and regex_dfas crates

[vorporeal]

Description

This PR continues the crate-split work that decreases incremental compile times for the warp crate. It extracts the secret-detection core into two new crates that sit below warp_terminal:

• secret_redaction — owns the compiled secret regexes (SECRETS_REGEX), the types SecretsRegex, RegexLevelMetadata, and SecretLevel, the setter set_user_and_enterprise_secret_regexes, the default patterns (regexes module), and the pure text helpers (find_secrets_in_text, find_secrets_in_text_with_levels, find_secrets_in_text_with_levels_using_regex, merge_sorted_ranges_with_levels). The string helpers and redact_secrets move here from app, and become plain pub functions.
• regex_dfas — owns the DFA-based regex search core (RegexDFAs, FindConfig) that was in warp_terminal's find module. This core powers all grid searching (find bar, async find, block filtering, and secret scanning). The grid-search loop stays in warp_terminal as GridHandler methods that take &RegexDFAs, because it depends on grid internals.

The PR also renames SecretRange to StringRange and moves it from warpui_core into string-offset. The type is a general "char range plus byte range" pair, and the new crates need it without a dependency on warpui_core.

What stays where:

• Grid obfuscation state (Secret, SecretHandle, SecretMap, IsObfuscated, ObfuscateSecrets) stays in warp_terminal.
• The view-coupled tooltip/span code and redact_inputs/redact_context stay in warp because they depend on app types.
• warp_terminal and app re-export the moved items from their old paths, so call sites do not change.

Note: this PR is stacked on #14875 (david/rss-shard-terminal-model).

Warp conversation: https://staging.warp.dev/conversation/024e797d-1d35-4c62-8394-abf93f7ddb0e
Plan: Crate-split scoping: shrink the warp crate

Linked Issue

N/A — refactor that is part of the crate-split compile-time work; there is no user-visible change.

Testing

• cargo check -p secret_redaction -p regex_dfas -p warp_terminal -p warp -p warp_tui --all-targets passes.
• cargo nextest run -p secret_redaction -p regex_dfas -p string-offset -p warp_terminal -p warpui_core — 867 tests pass. The regex pattern tests moved from warp_terminal into secret_redaction unchanged, and the word-boundary tests moved into regex_dfas unchanged.
• cargo nextest run -p warp -E 'test(/secret_redaction|redaction/)' — 40 tests pass.
• ./script/format and all three presubmit clippy passes are clean.
• No manual testing: this is a code move with compatibility re-exports, and behavior is covered by the existing tests above.

Agent Mode

• Warp Agent Mode - This PR was created via Warp's AI Agent Mode

CHANGELOG-NONE