Bump base64 0.22 -> 0.23 (Dependabot #166)

[l1a]

Summary

Bumps base64 0.22 → 0.23 — the one genuinely-new update from Dependabot #166
(the other 8 crates in that group already landed in #167/v0.6.3). It's a semver-breaking
0.x bump, so it was deliberately held out of the v0.6.3 consolidation pending an API check.
No runtime behavior change.

Compatibility check

base64 is used only under the optional graphics feature — src/logo.rs, two
base64::engine::general_purpose::STANDARD.encode() call sites that produce the inline-image
payload for the Kitty / iTerm2 graphics protocols. The Engine encode API is unchanged in
0.23, so:

• cargo build --features graphics and cargo clippy --features graphics -- -D warnings are
  clean (no deprecations). Note: the default gate/CI build does not compile base64 at
  all (it's graphics-gated), so this check was run explicitly with the feature on.
• cargo test --workspace and cargo test --features graphics both pass.
• cargo bench is unchanged — base64 is not on any benchmarked path (it only runs when
  emitting a graphical logo).

The Cargo.toml spec was widened "0.22" → "0.23" (a caret range wouldn't admit 0.23).

Versioning

retch-cli → 0.6.4; retch-sysinfo unchanged (0.1.46).

Test plan

• cargo build --features graphics — compiles clean against base64 0.23.
• cargo clippy --features graphics -- -D warnings and cargo clippy --workspace -- -D warnings — clean.
• cargo test --workspace and cargo test --features graphics — all green.
• cargo bench — core-probe benches unchanged (base64 off the hot path).
• just man regenerated (retch 0.6.4 footer).

Checklist notes

• Wiki (§4.8) not updated — no config/theming, module-layout, or just-recipe change.
• README (§4.5) not updated — no user-visible feature/flag/config change.

Follow-up

Closes out Dependabot #166. (After merge, #166 should auto-close as satisfied.)

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