feat(discord): track guild icon hashes

[Andiveli]

Summary

Move the unused guild icon URL helper and its focused tests out of the state-only boundary, preserving guild icon hash parsing and state behavior. The helper is restored in the final rendering boundary.

Why

Related to #332. The previous state head c432ee1 failed CI because guild_icon_url was not consumed there. This correction keeps the published history intact and aligns the helper with its first visible renderer consumer.

How

The new state head a291844 removes only the unused helper and tests. Compact, media, and render branches were advanced with normal merge commits, and render head 8927eeb restores the helper, tests, and existing re-export. No rebase or force-push was used.

Testing

• cargo fmt --all --check
• git diff --check
• State focused parser/state tests, 9 passed
• State cargo clippy --all-targets --all-features -- -D warnings
• Final focused URL/geometry/placement tests, 3 passed
• Final cargo clippy --all-targets --all-features -- -D warnings
• Final cargo test --all-features, 1,746 library tests passed, 1 ignored, plus 1 binary test
• Manual test in a real terminal (not applicable to this data-only boundary)

Chain context

📍 1. Guild icon state (#333, this boundary, head a291844)
   2. Compact sidebar rows (ab6aee7)
   3. Animated icon media (cfe8525)
   4. Animated icon rendering and badges (8927eeb)

Start: current upstream/main. End: guild payload/state and icon hashes are available. Follow-up branches own compact geometry, media loading, URL construction, and rendering.

Screenshots or recordings

Not applicable; no visible behavior changes in this boundary.

Checklist

• One logical change per PR.
• Link a related issue.
• No tokens, passwords, MFA codes, or raw auth bodies in code, tests, or logs.
• Change does not add self-bot automation, mass actions, or scraping.
• Updated README.md, if behavior or workflows changed (not required for this data-only boundary).

[Andiveli]

CI is failing on an unchanged upstream expression in src/discord/voice/dave.rs after stable advanced to Rust 1.91 (clippy::nonminimal_bool). The file is identical between this PR and main. I opened #334 with the isolated compatibility fix; once it lands, this PR can be rebased/rerun without mixing the unrelated change into the guild-icon state commit.

[Andiveli]

The branch is now updated onto current main through a normal merge, so CI should rerun. The three clean follow-up branches are prepared and published, but they will be opened sequentially after each parent merges to avoid cumulative fork diffs.